Why did Buddhism start declining in India in the early medieval times ?

1. Buddha was by that time considered as one of the incarnations of Vishnu and thus became a part of Vaishnavism.
2. The invading tribes from Central Asia till the time of last Gupta king adopted Hinduism and persecuted Buddhists.
3. The Kings of Gupta dynasty were strongly opposed to Buddhism.

Which of the statements given above is/are correct ?