Probably since the street power of the bikers among white criminals eclipsed that of the mob.

Make no mistake, their fashion sense may not be up to your taste, but the sense of money making some of these guys have is way above average. You may think they look like thuggish white trash, but they're savvy enough to be a significant force. Drugs, gambling, loansharking, prostitution, weapon trafficking, fencing...they have their fingers in all sorts of pies. And whereas the mob these days has the reputation to shy away from violence, the bikers still have the notoriety for using brute force when necessary - even if biker violence in reality is actually sporadic nowadays.