Because they have very promising potential and big populations to go along with it.
Imagine that out of 300 million Americans 270 million of them struggle to have two meals a day. What do you think our situation will be?
PS: When China and India reach the point where you can open your mouth in the shower when visiting these countries and not getting sick for two weeks, then it will be their first step of being called "developed nations" but they will still be far away from being called superpowers.
China is in a bubble right now with overinvestment from their currency manipulation. Rather than investing to improve their productivity they are investing in speculative stocks because the inevitable currency adjustment will guarantee a profit. After that, they will crash hard.