Fergie got him because he thought Moyes would listen to him. Fergie advised him to keep the back room staff for sometime. The first thing Moyes did was to sack them and bring in a guy no one has ever heard of as his assistant.
Then he over trained RVP so much that RVP pulled a muscle and got injured. He then started going after Fabregas, Bale, Ronaldo and Messi etc thinking they wanted to play for David Moyes. The man is delusional and the damage he has done in such a short time to United is still going on and United have still not received from it.
To make matters worse he cannot forget about it and thinks he is entitled to Manage United till his 6 year contract is over.
Furthermore, you cannot compare Moyes to Big Sam. Big Sam at least keeps teams up and have never been relegated from the PL. He has never been sacked for being relegation fodder either. He has always managed much smaller clubs than Everton before and now it looks like he is going to keep them up again while when he took over they look they might get relegated.