Actually I think it was when the WAC split and the MWC was formed - the lower conferences were "punished" for a new upper-mid major being formed. Since then the MWC has fairly remained status quo in terms of power while the WAC has become a lower conference as well as too many other things to keep track of, but originally the two lowest 16s were forced to play a play-in game because of that.
This is true ... that's why it went to 65 -- the "majors" didn't lose that coveted final at-large slot when # of leagues increased from 31 to 32.