This is a good question. I tend to think that even without Nebraska, Oklahoma doesn't get in. While Nebraska's academics are not up to Big Ten standards today, Oklahoma's are far worse. It is possible, even likely, that taking a borderline school in Nebraska sealed Oklahoma out, though. There is definitely a loss of prestige by taking Nebraska right now, just like there was when taking Penn State 20 years ago. You guys brought PSU along and they're far more respectable today than they were then, as Nebraska will be (hopefully sooner rather than later) but it may be too much to take on two "project" schools at the same time.
However, that's presuming academics are in any way steering this ship, and frankly, I don't think they are at all. If the B1G was more interested in building themselves up academically Nebraska doesn't get invited. Why would it? The University of Kansas is a more prestigious school (barely) than we are right now.
The driver in this situation is athletic revenue, and specifically football, and even more specifically, growing the BTN brand. Oklahoma can do that because, like Nebraska, they are a national rather than a regional brand. They bring eyeballs to televisions across the nation, and they help in basketball as well as football. Nebraska is a football-only draw, so OU is superior there.
But if we're just talking money, football money, TV money, then the obvious answer is Texas and Oklahoma can go to hell. Texas is the biggest money-maker out there, and when they pull their heads out of their collective behinds, they're a national TV draw in a number of sports, academically they are very similar to the bulk of the Big Ten, and while they're disparate culturally, they are not SO MUCH so that they cannot assimilate. There are myriad problems with having Texas in your conference, and I'm not going to beat that dead horse again, but if they can check their egos and play along, they could help the B1G and the BTN create a monster brand that would easily compete with the SEC/ESPN.