First and foremost, it's about not re-distributing value. These decisions are made by owners, and the owners know exactly how valuable each market is. That's why the NFC East is not likely to change … because the markets of New York, Philadelphia, Washington and Dallas are four huge markets. The owners paid top dollar not just for those teams, but those teams' values are established in large part because of the divisional markets in which they play. Each of those markets wants to preserve those other major markets.
Let me explain. So New York is THE biggest market, and the Giants are New York's #1 based on local, national and international valuation … and it has been since its inception. Every owner wants to be in the division with the Giants. But, if you cannot be in the Giant's division, they all want to be in the Jets division, because it's New York's #2 franchise with huge valuation as well. Philadelphia may not be the urban center it once was, but it's still a huge media market and a town that covers a vast area of influence, reaching well into Pennsylvania, New Jersey and Delaware. Dallas may be one of the biggest brands in the NFL, and it is also a very large market with one of the top 3 followings in the NFL, so it's another team that every owner wants in its division. So, while Dallas geographically doesn't make much sense, the increased value it adds to its division rivals makes it very desirable. There is no way any NFC East owner would be OK with replacing them with markets like Buffalo, Pittsburgh, Baltimore or Cleveland … it's simply not going to happen.
The AFC East is the 2nd best division from an owner's point of view. It contains New York's #2 market, plus Boston and all points northeast in the "New England" area. Naturally dominant. While Miami isn't in that league from a market standpoint, the solid influx of New Yorkers that re-locate down to South Florida create a natural rivalry. Plus the fact we regularly get Super Bowls, also makes us an attractive division partner for those two anchor markets. Buffalo … yeah that's the outlier. Indianapolis, who used to be in the AFC East was the other outlier, but their owner was OK with moving out because he did not want to compete financially with those bigger market teams … plus the fact he wasn't in the best graces of the other owners, so he didn't want to make too many waves when we had the last re-alignment. Buffalo has a slight region overlap with the NY Jets, so ideally the league would like to move Buffalo out of the AFC East and a larger market team, such as Atlanta, into the AFC East, but that's not likely to happen because Buffalo would be very vocal against it.
So, the regions are generally geographic … but the owners understand that the value of their franchises depend directly upon certain things such as their stadium solution, home city, and the division in which they compete. This is the biggest roadblock to any re-alignment. Then certain rivalries have been built up, such as Pittsburgh vs. Baltimore, Green Bay vs. Chicago, and Washington vs. Dallas.
Side note: In case anyone does not know the source of the Cowboys vs. Redskins rivalry, you may find this interesting. So, for a long time Washington was the only NFL team in the south. There were no other teams really in the entire southern half of the United States until you reached the West Coast. So, the Redskins had a HUGE fan base. But along came teams like Atlanta that took a huge chunk of it. The Redskins were still THE team for most of the south and areas like Texas, Oklahoma, etc. So there was a vote to allow the new franchise in Dallas and it was one vote short, obviously with the Washington Redskins voting against it. So, the owner of the upstart Dallas Cowboys franchise copyrighted a song that had traditionally been played at games in Washington … "Hail to the Redskins." Yep, the Cowboys copyrighted the Redskins theme, and did not allow them to use it … until they changed their vote, upon which they signed it over to the Washington Redskins. Needless to say, there were a LOT of reasons for the Washington Redskins to hate the Dallas Cowboys … beyond the whole "cowboys and Indians" mascots. That rivalry has remained intense ever since the first day the Dallas Cowboys became an NFL team.
Yeah, rivalries like those the NFL would like to keep in tact, because it makes rivalry games relevant even if the teams are not competitive.