No problem.
One reason they couldn't merge right away was because they needed the federal government to pass a law making it legal (exempting them from anti-trust laws). In order to get this exemption, Commissioner Pete Rozelle promised that if the merger was allowed, no existing professional football franchise of either league would be moved from any city. But, to be fair, he didn't double pinkie swear promise 'never, ever'.
They also had to allow time for some of the teams to make changes in or switch their venues as a team's regular stadium was required to seat at least 50,000 (no more Bears at Wrigley).