Because its mostly **** and no one wants to hear it. No one is forcing anyone to rap about anything, nor live their life the way they want.
The reason they're stuck where they are is because thats who they are.
Alot of indie bands and artists become successful if they produce nice music, without much or if any commercial assistance from larger companies.
This and because the ones that do go out form underground are considered sellouts and mainstream by the "fans".
so basically it's not the industry, it's the "fans".