Why did Buddhism declined and then almost extinguished in India?

I am currently exploring why Buddhism declined in my country India. Most of internet sites tell me that it was because of islamic invasion, but then hinduism survived it. In my opinion, it is easy to follow Hinduism where you just have to worship a diety for material gains. However to understand Buddhism itself you need certain amount of intelligence and life experience. So the Indian kings must have found it easy to back Hinduism to reign as it bolsters desires to rule and perpetuate. What is your opinion on this?