AirInsight last week published a report about the narrowbody market, especially about the 100-149 seat segment. What they found is that - in their opinion - the failure of aircraft in this segment was due to either weak OEM's (and I guess that this means that the OEM was undercapitalized) or due to the fact that the aircraft were no clean sheet designs optimized for that market segments but rather derivatives of (mostly) larger planes. As such a "shrink", an airplane carries a lot of weight that is not needed for it's role. For example the wings are too large and heavy, as they were designed for the original and larger aircraft.
The only aircraft that is currently in production and is a point design is the EMB190/195 family - where the EMB195 seems to be a bit underpowered in terms of takeoff length at MTOW, but the thrust is OK for typically flown routes below 1000nm.