IMHO, UT and OU think of themselves as big dogs and control the B12. Big dogs usually don't like to bruise their egos by becoming also rans in other conferences. I think they stay where they their mega egos can survive...B12.
Big dogs won't to run and fight with other big dogs. You actually described a bully running roughshod over lesser programs. The egos will actually drive the Big 12 over a cliff if Conference doesn't entice a couple PAC 12 teams to come over.