The basic reasoning is that they want to add 2 at the same time. 15 doesn't work. And this way, they can add of their own accord. If they were to add 1 and have 15, other schools would know they are pressing for another. You kind of show your hand this way.
Actually, I've been thinking; for scheduling purposes in both Football and Basketball,
15 is better than
14. With 15 teams, you can divide it up into 3 5-team divisions/pods/whatever. In football, you play everyone in your division every year, and you play play everyone in one of the other two divisions, giving you
9 conference games. While this would cause problems with deciding the 2 "best" teams for the conference championship, I think it could be worked out. With this system, you would end up playing the teams not in your division every other year, and cycle through the home&away with those teams every 4 years.
For basketball, you can keep the 3 5-team divisions/pods, play everyone in your division home&away for 8 games, and play the teams in the other 2 divisions once, for an
18-game conference schedule. Seeding for the conference tournament may be a little tricky, but the potential debates would be interesting.
SCHEDULING FOR 14 TEAMS
If you have a 14-team conference, scheduling is trickier; football, if you're going to have a conference championship game, it's probably better to have 2 divisions (instead of a single table). With 2 7-team divisions, it becomes much messier. If you play every team in your division every year, that's 6 games. You can choose to play either 2 or 3 games against the other division's teams, giving you 8 or 9 conference games, but then could end only playing the teams in the other division every 3-4 years (for an 8-game conference schedule) or every 2-3 years (for a 9-game conference schedule), and of course double that to complete the home/away cycle, if the schedulers are even bothering with that. Having a single table would provide some scheduling flexibility, but then you have the possibility(likelihood?) of not playing some teams for an even longer period of time.
A 14-team conference in basketball would probably be best served by going with a single table instead of 2 divisions; divisions would mean with the traditional divisional home/away schedule, and a single game against the teams in the other division, would mean a 19-game conference schedule, which I think most wouldn't favor. With a single table of 14, there would be a lot of flexibility to play either 16 or 18-game conference schedules, and the flexibility to have home/away with 3 or 5 other teams.
Just some random thoughts, looking at things from a different perspective; I don't know if scheduling has crossed the minds of the decision-makers in this whole conference carousel, but in my mind 14 teams isn't a very good number at all for football, while it's more workable in basketball. 15 teams is actually a pretty solid number for scheduling; 16 teams isn't as good I think, whether you do 2 divisions or 4 pods, but better than 14.