It might not only be about ability to handle the aircraft.
Flybe is a scheduled, passenger airline. They need a certain minimum commercial experience level in the RHS to support and assist the Captain - Not just pressurised aircraft flying experience up at FL250+, but passenger handling on busy four sector days with 1 hour turnarounds.
The Captain will need help from the RHS in managing the turnarounds, and thinking about missing pax, offloading bags, loadsheets, getting cooperation from busy handling agents, catering, wheel chair pax, tug, push back crew etc. - something you just won't get on the same magnitude with a Kingair or Chieftain* where a few pax are brought out to the aircraft together in a minibus carrying their own baggage, with some snacks in a cool box.
Such things can of course be learned, like everything else, but low cost companys cannot afford to have people who are getting to grips with flying busy commercial schedules, becoming swamped with ground handling as well, leaving the Captain to sort everything.
*(No disrespect whatsoever intended towards those who do fly such aircraft.)