My guess is
-McCarthy's star has faded. He knows he's not going to get any plum HC jobs with talent in place and/or roster control.
-Jerry offers him a chance to get back in the game, with a talented roster, AND an established quarterback, AND offers him playcalling duties.
-The only catch is, you have to work in the Jerry circus, which drives a lot of top candidates away.
-The Cowboys job requires you to basically be a lapdog, and McCarthy, for all his bluster and "tough guy" attitude, is basically a lapdog.
That's why we got rejected by all the top college HCs, and final-round interviewed the consummate owner lapdog himself, Marvin Lewis.