Most clubs who keep having managers who fail are usually poorly run clubs with poor players.
Or they're clubs who haven't made the correct managerial appointments.
Since Fergie left, we've made three. Moyes was never good enough, and never was going to be. LVG was a talented manager but also a stubborn one who, despite his credentials, had a managerial history which included crashing and burning in some high-profile posts. The third, Mourinho, arrived here after by far the worst half-season of his career. None of the three failures are exactly miraculous or completely unexpected, in that regard. There are internal failings that go beyond all three men, but all three men have also largely been responsible for their own downfalls, with tactical failings or outdated playing styles.