There simply aren't enough DSN antennas to support many more MER rovers along with the other missions like Cassini, Chandra, etc. We can build more antennas, but they aren't cheap. Like I mentioned in an earliet post, what we really need is something like the Mars Network communications relay system. Some of the proposals also provide for a GPS style navigation capability, something that could real useful for landers and rovers.

The other limitation is the number of JPL personnel to operate all those rovers. They're stretched pretty thin right now with just two rovers in action. JPL would have to train quite a few more personnel and perhaps set up additional control centers to handle as many rovers as you visualize.