They expanded with the schools they did at the direct request and guidance of their TV partners. That’s not a conspiracy, it’s a known reality.
What made those leagues strong originally was a combination of large population states or plethora of in-conference football talent, combined with power programs that were A) benevolent towards their less resourced partners, and B) desired their own fiefdom.
The Big 8/SWC and Pac 8/10/12 footprints were dominated by schools that had absolutely no sense of community towards the less resourced partners and weren’t all that interested in creating their own orbits. And in what was most important, they were based west of the Mississippi in places with smaller populations, less high end talent, and less national audiences.
Between the demographic advantages of the Big 10/SEC footprints, and the unified fronts of their power programs, they were the better flagships for the TV powers to propagate