Because eating with your fingers makes you aware of the temperature of the food.
Otherwise you will burn your tongue and lips!
And some very high class people think that this the uncivilized and unclean method to eat food.
Tell me, can you determine that the food is hot with the help of a spoon? And how will you cut the roti with the cutlery?
The best way to relish INDIAN food is to eat with your bare fingers. And yes, don't forget to wash your hands before dining.