The biggest motivating factor seems to have been money. ESPN and Fox have the television rights for the conference currently and they were looking at the list of candidates and just not feeling. I'm not sure if they've closed a deal to chip in some more dinero for the conference not to expand, but the word on the street seems to be that they've dangled that possibility in front of the Big 12 and it seems to have worked- at least for now.
The second (and more interesting factor) to consider is the issue of politics. Here's the official rationale from The Commish himself this past July, saying that the conference would be looking:
"for members that will grow over time as we grow-- [schools] that bring stability, that have a high top end."Then, pretty much as soon as the process started, Texas politics muddied the waters by pretty much insisting that whomever the Big 12 added one candidate just had to be Houston. Now, Houston is a pretty solid candidate all by themselves, so you can't strike them from consideration, but with their inclusion in a potential expansion pretty much a fait accompli right out of the damn gate, suddenly you go from looking for schools to improve your brand, have a high top end and stabilize your conference to Houston + 1 (or 3, depending on how crazy you wanna get.)
In other words, the non-Texas schools are suddenly looking at this thinking, 'well, fuck. We gotta play nice with The Longhorn State and dilute our main recruiting ground or jump off this pony altogether' and they seemed to have decided to jump off the pony instead. The fact that their television partners weren't crazy about the idea either- and in fact, were so not crazy they were willing to pay them not to add members was probably all the excuse everybody else needed to kick the can down the road a bit.
But here's the thing. If a week is a long time in politics, five years is an eternity in the world of conference re-alignment. Could be that the Big 12 finds a formula that works for them and expands or doesn't expand and it's all good. But the general consensus is that by the time the current Big 12 grant of rights is up in 2023, people will be heading for the door and the era of the four 16-team super conferences that has long been predicted will come to pass. So, where does everyone go? Let's break it down:
Currently, the Big 10 and SEC sit at 14 members. The ACC has 15 and the Pac-12 is sitting at a good old solid 12 and let's throw in the Mountain West for the sake of argument-- that's got 11 members right now. So, if I'm doing my math right that's ten slots for ten schools- though if the music stops and someone gets stuck with the Mountain West and their chair they may not be all that happy about it. I'm leaving politics out of these scenarios, though you can best believe they'll be in play all over the place if the Big 12 really does start to fly apart.
Scenario 1 (The Hawkeye Nation Plan):
Oklahoma/Oklahoma State go to the SEC
Texas and Kansas to the Big 10
I'm assuming under these conditions the Pac-12 which was ready to blow up the Big 12 in a huge-ass way, goes buckwild and makes a play for Baylor, TCU and Texas Tech and maybe snags Kansas State along the way. This leaves the ACC to grab West Virginia (a logical choice for them) and Iowa State as the odd man out, probably headed to the Mountain West (where they might actually do really well.)
Scenario II (My Plan)
If demographics is destiny, then despite a good academic record, Kansas has nothing that the Big 10 really wants. Texas does, but the other school that doesn't get mentioned all that much, but had some buzz during the last round of expansion was Georgia Tech. Both of these options would end the Big 10's contiguous geographic footprint but plant the flag of the conference directly into two of the biggest football recruiting grounds in the nation: Texas and the Deep South. If the Big 10 can do this, I think they will. Which means:
Texas to the Big 10.
Oklahoma and Oklahoma State to the SEC (we're assuming they're a package deal, which is probably the case.)
Kansas and West Virginia to the ACC (If the Big 10 snags Georgia Tech, they'd be fools not to take West Virginia and Kansas, the former a solid basketball and football program, the latter a home run in basketball and solid academics. But if Oklahoma is a package deal, are the Kansas schools?)
The problem for the remaining five schools in this scenario becomes the Pac-12: Tech, Baylor, TCU, K-State and Iowa State aren't exactly barn-burners when you have BYU, Air Force and potentially Colorado State to consider. While the service academies always get mentioned in scenarios like this, they never seem to get taken on by the Big Players and they've already got Colorado- so why do they need Colorado State? Honor code issues aside, BYU would be a solid add for them. So then, it's (probably) BYU + 3. Problem is, which 3? I'd say TCU and Tech are strong candidates- Baylor has a public relations problem which you'd hope would be behind them by 2020. Maybe you go buckwild for Texas and take all 3. Which leaves ISU and Kansas State out in the cold, probably headed to the Mountain West.
As per usual, all these predictions are totally and utterly dependent on the Big 12 flying apart in the first place and will probably wind up being totally wrong. But now, I've got to put aside my excitement and wait for the Beast to rear it's ugly head again... but no one is talking about the darkest horse of them all.
(Addendum: This and this are pretty decent takes on this. h/t to The Quiet Man for the first link which is absolutely 100% correct.)