Things happen faster, and not everyone can get used to it.

Also jets tend to be bigger, and thus have more systems with more gee whiz.

Jets tend to have more advanced avionics capabailities, which means more procedures and profiles to learn.

After lengthy observation, not many prop pilots fail regional training (assuming good attitude and work ethic) but those that do tend to either have trouble keeping up with the jet, or just can't get their head around all the stuff they need to learn.

Airlines who can afford to be picky usually prefer applicants with some jet time. In that regard it is like riding a bicycle.... if you can learn one jet you can probably learn other jets.

Regionals are entry level jobs, and all "jet" jobs now. Filled with the least experience, it's one of the safest places to put a no-experience know-nothing new-hire.

Given a choice between someone with a little "jet" experience" and someone who's been slogging out the weather in a low-performance piece of junk at night, single pilot in ice and heavy traffic, I'll take the second guy. Bonus if he was flying something that required some forethought and planning and skill, instead of throwing the thrust levers forward to takeoff, and idle descents.