Why is Illogic an Underground Rapper?

May sound like a stupid question but I want to know the reason I recently start listening to underground rappers and he is one of the best I've heard him and Aesop, MF Doom etc are better then most mainstream rappers and Eminem seem to have fallen off his throne I think Rap might actually be dying(or waiting for a rebirth).... I digress I just wanted to know why stay underground? With skills like his he would be compared to some of the more well known rappers but I would still consider him better