They, with the possible exception of Noah to some degree, were not really built for in depth characterization or to last over seasons.
I would also add that Mohinder, before he went all Seth Brundle/mad scientist like, had the potential to ladt a few seasons. He had a more sympathetic, yet limted, understanding of what the supers were going through. He could have been just as good, if not better, character than HRG.