Please don't take my comments the wrong way. I in no way am suggesting a dash pilot or any prop pilot for that matter cannot fly a jet, that would be a ridiculous statement.
What I said was that they are different animals and it takes time to adjust, either way. I.e. Prop - jet or jet to prop. More training = more money.
Speed has little to do with it either.
below 10,0000' the speeds are mostly the same, and that's where it all happens.
it's a series of issues, weight, drag and system complexity probably the main ones.