1. Decline of royal patronage
The royal patronage of power kings contributed towards the growth of Buddhism in India. It had to fight hard with Brahmanical religion.
2. Impact of Sankaracharya’s Preaching’s
Sankaracharya was the great Hindu saint who reinforced the principles of ancient Hindu religion. Many scholars believe that the Brahmanical revival under Sankaracharya led to decline of Buddhism in India.
3. Internal weaknesses of Buddhism
It is reasonable to suggest that Buddhism declined more for its own failures than for any external causes.
3.1. Hinayana or Theravadins advocated a dry, rational, monkish creed which gradually lost. The common men failed to comprehend the philosophical aspects of Buddha’s teachings.
3.2. After the death of Buddha, various sects arose in the Buddhist church. They were opposed to each other and each of them claimed to be true followers of Buddha’s doctrines.
4. . Assimilation with Hinduism
In later periods, it was preached that worship of Buddha will bring salvation rather than the practice of the ethical doctrines the eight-fold Paths. Such a great departure from original Buddhism caused a great blow to the survival of the creed.
5. Foreign invasions and attack by hostile forces
When Buddhism was in such a state of degeneration repeated attacks by hostile force and foreign invaders totally uprooted the creed from India.
6. Socio-economic cause
D.D. Kosambi has given a socio-economic explanation for the decline of Buddhism in India. The land system and the structure of land holding in India underwent a transformation.