If there aren't any reward fares available then they won;'t show you a price for any mpm fares (as they are classed as the same). LAS is a very hard route to get reward fares on.
Point to note, I have done two MPM fares within the last year to LAS in Feb/March and they werent that much cheaper (£100) than the lowest miles earning fare. Both times have been sell outs and even super bowl weekend.
I only do MPM to use my miles as I only fly twice a year, if your a mileage junkie then there not always the best option.