I'll tell you an inside secret why people prefer folks from IIT/IIM.
It's because of their perspective, risk taking ability and aspirations. People from tier 1 clg generally aspire to become big and they are intrinsically motivated to do whatever it takes to achieve things.
They have a larger perspective towards life since they are exposed to a lot of things in college.
And that's it. I'm not saying that all IIT/IIM folks have this, or other folks don't have it. It's just that IIT/IIM graduates have worked hard to get there and already proven that they have that fire.