When we're talking about Hollywood, we usually talk about big budget movies. Studios fund these movie projects not out of love of art, but to make a profit. To make a sizable profit they need to sell the movie to the audience so that they buy tickets (and buy DVDs and BluRays ... etc). One of the bigger selling points of any movies is the cast. People will pay money to see their favorite actors on screen. That's why actors with greater popularity (even if they don't really act that well) can demand bigger salaries.
Now, every country has its own cache of superstar actors that people in their own country will recognize. But very very seldom does this recognition cross borders. If you ask any Indian (or fan of Indian cinema) who Amitabh Bachchan is, they will most likely know who he is. Now ask that same question to anyone else, and they'll probably have to Google the answer. Thus, Amitabh Bachchan can sell movies to Indians and fans of Indian movies but probably not to the rest of the world.
It's not a comment on their acting abilities, but on their selling ability.
Another thing is that Hollywood casting is still pretty much stuck on stereotypes. When Hollywood casts an actor of an ethnic background, they will usually stick that actor with a role stereotypical to his ethnicity. So when they cast an Indian actor, it will be to fill a certain type of role instead of a generic one. Thus, Ashwariya Rai as an Indian girl in an arranged marriage, Dev Patel as an Indian slums boy .... etc. That stereotyping further limits the role of Non-American actors (not just Bollywood actors) in Hollywood.