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?
The religion which has committed greater sacrifice for the humanity will always come on the top. Good Karma never dies. Moreover we believe in the cycles of period. This period belongs to Christianity. May be next will belong to Hinduism and then may be Buddhism will come. In this era the things are happening exactly the way it should. Only a fool will believe that Greatest religion now will remain the greatest religion forever because Sabbe Dhamma Anatta. 
Buddhism declined the way it should have declined for the conditioning of this era. Give up these mental fabrications. These are not good for Anatta.