In my opinion the current situation has been caused by four main factors;
Allowing Fergie to name his successor before his departure. Mourhino was the right man for that job and everyone knows it. I'm not saying that Moyes is a bad manager nor do I blame him for the current situation but don't think he was the right choice.
A lack of funds to compete with the other big clubs. Football is all about money now and United need about 200 million to spend on players and strengthen their squad. They need three defenders and three mid-fielders. If they don't spend that much then they won't improve because the players that have come through the youth policy in the last few years aren't good enough in my opinion. Considering how much debt the Glazers still have left to pay, it's unlikely that they have that much to spend on new players.
Fergie didn't buy well in the transfer market in the years preceding his departure. They did sign Van Persie, and Kagawa but apart from that they haven't brought in enough quality players which has cost them. For a club of United's size their dealings in the transfer market in the last four years have been quite pathetic. On the whole the current United team is coming to an end, the likes of Giggs, Ferdinand, Evra and Vidic will be retired soon and nothing was done to bring in replacements for these players.
Not enough quality in mid-field. This is their biggest problem because without good mid-field players you're not going to create anything. I look at their mid-field now and it just doesn't compare with the other big clubs.