my guess is that it has to do with running the business of the NFL & the value of individual teams. I would think it would give leverage to teams if they can always threaten to move, ie- getting new stadiums. Say for example, the WNY public bought the bills, and refused a new stadium, and ran the team into the ground. Or any city/team for that matter. Seems like the value of the team would plummet, and likewise hurt the NFL.
Maybe jacksonville is better example, if they were stuck in jacksnville, the team as a business may not survive.
Just a theory, I'm no business expert, but my guess is $$$ is behind that ban.