The issue with BB is the same as with every other reality TV show - the appeal lasts as long as the individuals are in our living rooms regularly - and fades really quickly when they cease to be presented in the way we originally saw them.
Thus, although Nikki got her own show - essentially being put in different situations where she can throw tantrums, the appeal is limited because there is not interaction dynamic over a long period, so the show has seriously limited appeal.
The remainder of the people have even less chance of developing any career in the public eye, so most return to obscurity, as is the nature of their appeal - or indeed not, for a lot of people.