Because Texas and California are such large states that artists from those states can eat just as good as national mainstream artists, if not better, off of only having local fanbases.
Texas is one of the pioneer states when it comes to the independent rap game (Rap-A-Lot, Suave House, S.U.C., Swishahouse). Z-Ro saying he makes $10K a show just performing at Texas nightclubs and frat parties. Slim Thug had a Bentley and a mansion before Pharell signed him.
Who needs a national fanbase when you got it like that?