To succeed in Bollywood, you need to have a certain look. That is, you have to look almost like a white Caucasian with Indian touch. If you do not possess that, you have to be a Punjabi
Actors from South cannot succeed in Bollywood as most do not possess those looks. Even if someone from south do have those fair skin Caucasian looks, the stigma of being from south will always haunt them. This only applies to Heroes. For Heroines, it does not matter. Bollywood audience do not care which part of the country the leading lady comes from as long as the girl is good looking.
Also, the masala that works in South Film industry will not work in Bollywood. In South movies, the heroes are always elevated to such an extent that one would think if this hero can kill superman with bare hands. Heroes can do weird stunts which defy all logic and get away with it. Worst thing is, the front benchers in the theaters love those stupid stunts. Those are the people the producers of the movie target. A few stupid stunts and a couple of Item songs is what they look for.
In Bollywood, you do not see such idiotic stunts, but you see pathetic comedy interlaced with melodious music where the chocolate boy hero dances with a milky white heroine with a few blonde models in the background. The typical South Actor's image will never fit into such roles.
There is one thing in common with all Indian movies though. Most directors or story writers do not waste time in trying to be innovative with the story. They producers of the movie want the movie to be done in 2 or 3 months. So the writers either copy the story from some Hollywood flick or just give a new twist to an existing story. Audience are dumb. Most do not care about the story. They only look for some sex scenes which the heroine or some item girl will provide.
If the producer is ready to invest 5 crores, he expects the film to make 6 crores with 1 crore profit. A quick buck in less than 6 months time. Better than putting your money in Share Market
Bottom line, Indian audience do not care about the quality. Even though most films are flop, the producers make their money in the opening 10 days or they at least break even.