Lots of factors but among them is the ridiculous first year wages at some of the ULCCs. There was a long discussion on this subject on a Frontier thread back when their negotiations were going on. The pilot group didn't wish to "waste" any "negotiating capital" on bringing newbie's on board. So what did the first year FO wages end up?

$58 an hour for Frontier
$57 for Spirit
$78 for JetBlue

So if you are currently making $90-100 an hour as a regional captain, WITH YOUR CHOICE OF SCHEDULES, how hard is it to take a 45% pay cut for a year and go back on reserve in some crash pad for a couple years, because your roots are too firmly planted after a decade or so to relocate? Pretty hard. Not impossible, certainly, but it's a serious emotional barrier, and the more senior (i.e., older) you are, the smaller the carrot for doing it and the greater the carrot for just staying put.

So in a way it's the major airlines' pilot union negotiating tactics that are hurting these guys, or at least making the decision to move on more complicated than it would otherwise be.