Large turboprops like the Q300/400 and ATR *should* be making a comeback, as the hub/spoke system (especially east of Kansas City) makes *perfect* sense for may destinations from a stage length and cost standpoint.
I would guess one reason they have not, and its a big reason, is there are hundreds of 50 seat RJs out there that are largely paid for or have fairly low lease payments compared to the cost of capital to acquire a fleet of new large turboprops.