Bananamanuk,
The Atr 42-500 has a great performance for a turboprop and a good TAS too but it is simply too expensive running on RE's network compared to a 72. The ATR45 has almost the same fuel flow as the 72 and takes 20ish pax less, not viable for RE's relative lower ticket prices. If you need new planes like RE believed a few years back then I think they made the right choice by only getting AT75 and use the older AT43 on thinner routes.