Ferguson was successful because he made the most out of what he has.
At Aberdeen, he got them to win the Scottish League when Celtic and Rangers were dominating, while also winning the secondar Euuropean competition at the time (Cup Winners Cup)
Has come out on top in the Premier League with United despite numerous challenges (Arsenal in late 90-s-2004, Chelsea, Man City) with supposedly inferior teams. He's a very ambitious manager. When you play at Old Trafford, you know United are going to get at you, regardless of the opposition. Despite the apparent gap between the squads of United and Madrid, he nearly knocked out Real Madrid from the Champions League.
David Moyes is a good manager, but what Ferguson has that Moyes doesn't is a lack of fear and ambition. Ferguson would rather risk losing in order to win, whereas Moyes doesn't like being defeated. As a result, while Ferguson may lose more games, he'll end up picking more points by having the ambition to win matches. If you settle for a draw over 3 matches, you'll get 3 points. If you go for games that are tied without fear of losing, there's a 60% chance you win if you're Manchester United. SAF would always go for the win because as United, you know that in those knife edge games, they're going to get at least 4 points from 9. 4 points out of 9 is better than 3 and often in Fergie's case it would be 6/9, and that's what made him so successful.
If Moyes can develop that ambition and risk-taking (which it really isn't in reality), then he can become succesful. Until then, he's just a manager at a big club with a little club mentality.
(Chelsea fan)