I could give you a very long explanation as to why there are very few operators of 19 seat aircraft
Because the manufacturers, in the majority of cases, stopped manufacturing them!
Such types as DHC6, BN3, Do228, Do328, SF340, S2000, J31, J41, Shed330, Shed360, Bandit, Brasilia, Metro, Beech 1900 all went out of production.
The DHC8 started off as a 36 seater but became stretched, then stretched, then stretched,the ATR42 (whilst still in production) became the ATR72 and the HS748 was another one that grew in size.
At least they've re-started production of DHC6, Do228 and, perhaps by now, Do328 and god forbid should they ever reproduce the Shed330/360
As for a replacement for an IOM operator all it may take is for someone to order, perhaps, a few Do228/328's, let's say base the company in Wales for some Welsh government handouts, and take it from there, the group trying to reopen Plymouth Airport claim to have such a proposed operator lined up so it can't be that difficult.