First off, LSU is absolutely in a big TV market…so that’s not the greatest example.
You need to substitute “TV market” for “likely subscribers”. There’s some overlap there, but it varies from school to school. The biggest correlation is with historical success and number of living alumni. To your LSU / Northwestern comparison, LSU dominates them in both. Now, substitute Northwestern for Illinois, and suddenly its much closer. Illinois’ football success of course doesn’t touch LSU’s and is really no different from Northwestern. But they are a much, much larger school with many more living alums. For them, that Chicago market sure as hell matters.
Virginia vs. Clemson is also more interesting than you think. Clemson had a nice run with Deshaun Watson and Trevor Lawrence, but they never were and never will be a blue blood. Their time in the sun is over. Take out that half decade, and there’s not a lot of difference at all between them and Virginia, from a national interest perspective. Virginia’s another large school with huge presence in that DC market, which matters. If Clemson was an AAC institution, with 40,000 students, and the SEC didn’t already have a foothold in South Carolina, then they would be a far more interesting prospect for both the SEC and B1G. But none of those 3 things are true.