There are several factors that led to the decline of Buddhism in India. Some of them are as follows:
- The decline of the Gupta Empire led to the end of state patronage to Buddhism
- Buddhism in India was a monastic movement. As such it lost support of its lay supporters.
- Corruption of the Buddhist monastic order contributed to its decline in India as the monasteries became wealthy enough to not adhere to the core teachings of the Buddha
For further reading check the following articles:
- Differences between Buddhism and Jainism
- Differences between Mahayana and Hinayana Buddhism