It's probably a combination of low pay and not enough certified pilots. Right now it mostly has to do with low pay. There are pilots out there that just don't want to put up with the regionals, if you go to your local fbo or flight school, most CFIs want to work charter or corporate. However if things don't change we'll probably enter the 2nd phase of not having enough pilots even with good pay. The last 5 years or so, there was a big drop in certified pilots, I guess the world wide pilot shortage is catching up here in the U.S.