What links three of the four teams left is that they're based in southern states.
Florida, Dallas, and Las Vegas all have no state tax. So the players don't pay taxes when they go to live there and play, which is about 20% of their salaries.
World Financial Report: 3 of the 4 teams left have 0 state tax. Not a coincidence, such a clear advantage to be able to pay guys 20% less against the cap.
It looks like southern teams have a clear advantage in attracting players to their teams. In addition to the lifestyle and the hot sun, they simply pay less money to live.