The problem is with certification and development costs. If Boeing were to get firm orders for, say, 50 of the 711, the might do it. But right now, the CRJ-700/900 and the larger ERJ-145 (and the larger variant the designation of which escapes me at the moment) are filling the market for a 70-90 seat aircraft.
Also, landing fees would probably be higher for the 711 than the CRJ-900, so unless someone is looking for fleet commonality requirements (say FL looking to operate its own JetConnect service instead of Air Wisconsin operating it), then I could see Boeing developing the 711, 712ER, and maybe even a 713 in addition to the current (and only) 712. But right now, FL and the other 712 customers seem happy with what they have.
Oh, one more thing: The current 717-200 used to be the MD-95-30. The MD-95-10 was going to fill this market. Boeing looked at the in-production CRJ and the upcoming ERJ and decided not to try to compete with them. The MD-95-50, what would now be called the 717-300, was on the board and I assume is waiting for an airline to give Boeing a firm order, though it's unlikely this will happen any time soon.
Those who fail to learn history are doomed to repeat it in summer school.