Answer:
they were decided on a couple things. primarily location. its north south east west. unlike the nba, where the Memphis and minnesota are western conference, and mlb, where moving the astros to the al west would make all divisions equal yet they chose not to (don't get me started with nhl), the nfl has done a good job putting teams in realistic regions. they also kept rivalries in mind. they wanted to make sure titans jaguars stayed together, and many other rivalries. the 8 divisions help teams and fans keep playoff dreams simple. there are 3 other teams to compete with for one spot. it really has helped enhance team rivalries and teams that have not mattered much to each other now find themselves archrivals.