Have listed six reasons plus the words of Swami Vivekanand & Dr B R Ambedkar.
1. The main cause was the neglect by the monks of this life and its values. While the Buddhist monks realized that everyone was not fit or could not become a monk or nun, they paid attention only to the life of a monk and not to the life of a householder. Which meant that they focused on the life of a monk, which is a life of inwardness as compared to that of a householder, which is one of outwardness. Now, both these aspects need examination, study, guidance and control. It is not enough to tell a householder that this existing life is only a stepping-stone to the life of a monk. Why and how is it so and what relation it bears to realities has to be explained. Instead Buddhist philosophers began to teach that this life was nothing but a value of tears and misery. While some forms of Vedanta taught the same philosophy, the attitude of Mimamsa (philosophy of action) and the Epics saved Hinduism from the fate that overtook Buddhism in India. Many great Indians were impacted by spiritual teachings but "unless there were some codes extolling the values of the world, they tended to become one-sidedly inwardly ".
2. Another reason was the admission of women into monasteries and the more or less indiscriminate conversion of men, women into monks and nuns. While true renunciation and celibacy were appreciated, people wanted to see them well practiced. When people supported these monasteries with their hard-earned money, they did not want its residents to live in luxury and enjoyment, virtues, which were condemned. If monks and nuns had lived by the rules that they were taught, people would have supported them inspite of any hardship that they had to face.
3. The next reason was the deterioration in the political and economic life of the country. Monasteries were supported by the people and the Kings e.g. Ashoka. Now, when a dynasty fell or a king died, the next in line might not give the same degree of support. The king's thinkers realized that their defeat was due to the loss of their best fighters, leaders, who had become monks. This made the country an easy prey to the foreign invader. Coincidence or otherwise, India's first foreign invasion by the Greeks took place in 327 B.C. a couple of centuries after Emperor Asoka's peace movement.
4. Buddhism existed in the monasteries and unlike the dharmaasutras (ethical codes) lacked a moral code. So when monasteries disappeared, Buddhism disappeared. The invasion of the Muslims and the ruthless destruction of Buddhist monasteries extinguished the lamp of Buddhism in North India. The wanton destruction of the great monastery of Uddandapura (Bihar) and the wholesale massacre of its monks might make us visualize how the great monasteries of Nalanda, Vikramasila and others met with a tragic end.
5. The extreme asceticism practiced and popularized by both Buddhism and Jainism disturbed the social life of India. Magadha, the seat of many imperial dynasties, became Bihar, the land of monasteries (viharas). There was nothing in these religions to emphasize the importance of life in this world and its values. These causes led to a bloodless revolt by the orthodox in the eight-century a.d. The revolt was staged from two sides, the Brahmanic and the Upanisadic. Kumarila was the leader of the former and Sankara of the latter. Kumarila succeeded in reviving a strong positive attitude towards the world and its values and all that could be called human and activistic. On the other hand, Sankara said that everything that was good in Buddhism already existed in the Upanishads. In fact, Gaudapada, the grand teacher of Sankara, unified the current spanda (vibration) doctrine of Saivism, the vijnana (mind) doctrine of the Buddhists and the Atman doctrine of the Upanishads in his Mandukyakarikas and made the way easy for Sankara to assimilate and absorb Buddhism. Thus, there remained no justification for its separate existence in India; it had no social ethics and consequently, no hold over society. It could not stand alone as a spiritual discipline as it was shown to be part of the Upanishads.
6. Quoting Swami Vivekananda " Thus, inspite of preaching mercy to animals, inspite of the sublime ethical religion, inspite of the discussions about the existence or non-existence of a permanent soul, the whole building of Buddhism tumbled down piece-meal and the ruin was simply hideous. The most hideous ceremonies, the most obscene books that human hands ever wrote or the human brain ever conceived, have all been the creation of the degraded Buddhism. The Tartars and the Baluchis and all the hideous races of mankind that came to India, became Buddhists and assimilated with us, brought their national customs and the whole of our national life became a huge page of the most horrible, bestial customs. Sankara came and showed that the real essence of Buddhism and that of Vedanta are not very different but that the disciples did not understand the master and have degraded themselves, denied the existence of soul and one God and have become atheists. That was what Sankara showed and all the Buddhists began to come back to their old religion".
7. Buddhism adopted various thoughts and beliefs between the first century B.C. and the sixth century a.d. Some Buddhists adopted the tantric sadhanas and distorted them for the sake of enjoyment and comfort. The highly advanced philosophy of tantric sadhana is difficult to understand without the guidance of a proper teacher. This undigested knowledge of tantra, including the use of wine, meat, fish, gestures and physical union led these Buddhist followers to their downfall. Also, the distortions of Buddhism produced a variety of schools, which were not pure Buddhist schools but contained a variety of practices. To give you an idea of the syntheses between Vedanta and Buddhism, the concept of Maya in Vedanta in borrowed from Buddhism. Sankara accepted the logical connotation of Maya just as it was given by the Buddhists. Jainism was saved by tacitly allowing its members to become part of the Hindu fold by adopting rules of conduct of the third caste, namely Vaisyas or traders.
8. Quoted from 'Dr Ambedkar Life & Mission by Dhananjay Keer'. Dr B R Ambedkar addressed delegates of Young Men's Buddhist Association in May 1950 at Colombo on 'Rise & fall of Buddhism in India' - 'Buddhism in its material force had disappeared. But as a spiritual force it still exists'. As regards Hinduism he said it went through three phases, Vedic religion, Brahmanism and Hinduism. It was during the Brahmanism period that Buddhism was born. It was not true that after the days of Shankaracharya Buddhism was dead in India. It was going on for years together. In fact Shankaracharya and his teacher were both Buddhists he added. While he was digging material on the subject for the decline/vanish of Buddhism from India the reasons were - adoption of some rituals & practices from Buddhism by the Vaishnava & Shaiva cults, which were vociferous in their propaganda against Buddhism. During the invasion by Allauddin Khilji thousands of priests in Bihar were massacred and consequently some of them fled for their lives to Tibet, China & Nepal. In the meanwhile, the majority of Buddhists went over to Hinduism. The third cause was that Buddhism was difficult to practice while Hinduism was not. Reason four was that the political atmosphere in India had been unfavorable to the advancement of Buddhism he concluded.
But according to Hindu scholars the fall of Buddhism was due to many reasons. Owing to universalistic ambition its spread was everywhere but it had geographical center nowhere. It discarded all national gods & godmen & proclaimed Buddha the greatest of all gods. As long as it reacted as a reformative flank in India, Buddhism gained ground but when it began to act against the Vedic religion, which was the national religion of the majority, Buddhism lost sympathy in India. The Vedic Hindus fought the Muslims bravely and did not flee to any other co
untry. But the Buddhists when attacked, having a center nowhere, fled to different countries and even it is said acclaimed the invasion of India by non-Hindus with the ringing of bells. Besides its godlessness, its over-emphasis on redemption, its sad tone, its unconcern with the world & neglect of family checked rather than fostered enterprise. Quote ends.
Books referred to 1. Introduction to Comparative Philosophy by P T Raju. 2. History & Culture of Indian People by Bhartiya Vidya Bhavan.
Long Live Sanatan Dharam