Why do light from two different sources doesn't form coherent sources?

Why would they? It makes more sense for them to be different. There is not an equal amount of energy in the production of the light, they are probably different colors, and if they are not in an identical place to begin with, they’ll be out of phase.

If you study how coherent light is generated, then you will answer your own question.

Welcome to Fluther, @vamsiviswa. Here’s a good discussion of producing coherent light. Read it and you will instantly grasp why light from to different sources will be incoherent.