Bill Belichick left the Jets because his contract promised him total control over personnel when he became head coach. Parcells threw him a curve ball when he stepped down as head coach and then announced he would stay as general manager overseeing personnel and Belichick bolted no longer willing to work under Parcells.
In a review of the new David Halberstam book on Belichick, Providence Journal writer Bill Reynolds describes a situation in the book where Belichick was with Parcells in New York. It was during a game. After a heated exchange over the open coaches microphones, Parcells said to Belichick, "Yeah, you're a genius, everyone knows it, a ......... genius, but that's why you failed as a head coach--that's why you'll never be a head coach. Some genius."
Who wouldn't want to work for a guy like that?