I made the move from Q400 to 737-800 around a year ago. Very fortunate that my new employer does not discriminate against TP drivers, and their requirement is simply 2,500hrs TT and 1,000hrs EFIS Multicrew >10t MTOW. And they pay for the type rating.
Frankly, the 737 is a much easier day out that the Dash. It's smoother, quieter, above most of the weather most of the time, and best of all it has hot food. It might have a comedy overhead panel, but apart from that it's easier to operate, has fewer gotchas and isn't constantly trying to give you an altitude bust. Granted the FMC is more sophisticated, and the choice of VNAV, level change and V/S give you more ways to cock-up the descent, but once you've got your head round the systems, the only real challenge compared to a TP is getting it to slow down. And in the sim, it's an absolute pussycat on 1 engine compared to the Q400. The only time I'd take a Dash over the 737 is in gusty crosswind conditions - other than that, it's all good.
Why do many airlines disregard TP experience and insist on jet time? I really can't think of a valid reason. If you were transferring from something ancient like an F27 or a Shed, the transition to EFIS / FMS and much higher speeds might be a big challenge for some people, but the likes of the Q400, Saab 2000 and newer ATRs have full glass flight decks, and in the case of the Dash and Saab, they are pretty quick as well.
Alpine Flyer is probably right about the market reality. If airlines have enough decent applicants with jet experience, some might prefer not to bother with TP pilots because of a perceived training risk from the 'step up'. IMHO, the risk is no different to anyone converting from another type, and turboprop experience is just as valuable as jet time.