IIRC, F9 was flying the larger planes to ABQ back when they were still using Mesa as their RJ partner. These regional jets only held 50 and with the onward flight to ELP they need the greater capacity than 50. Currently, with the short length of the flight and the ease to get to Denver for connections I would not see a larger plane over the 70 seat CR7 any time soon. Based on the Feb T-100 the load factor was at 63%.

Regarding Mexico, I don't see enough O&D demand for that service currently. The Frontier of the 80's was a different company than the current Frontier Airlines.