There are multiple factors. But the biggest is that GM Belichick undermined Coach Belichick with his terrible draft picks and free agent signings. Even so, ESPN analyst Bill Barnwell makes a good case for NOT firing Belichick right now (though maybe they should limit him to coaching only).
It’s now fashionable to say Brady, not Belichick was responsible for the great Pats dynasty. It is indeed true that Belichick has never won much without Brady, while the reverse is a different story. But it’s also important to remember that great defense was a key to the Pats dynasty, in addition to Brady’s passing. And that defense was built by Belichick. Brady didn’t contribute much on that side of the ball, except by keeping the opposing offense off the field.