Not just the manager

Since Sir Alex Ferguson retired, there have been several factors causing a comparative malaise. Yes, they've won three trophies since 2013 - the FA Cup, League Cup, and Europa League.
But they didn't plan for life post- Sir Alex Ferguson. The club was geared towards his domineering presence and success, and the structures in place couldn't account for his departure.
Moreover, appointing Moyes and van Gaal were clearly errors, and both were not attuned to getting United back as a winning club.
A major facet though is the Glazers. Yes, success was attained under Sir Alex, but largely in spite of the Glazers, and not because of them. The Glazers have allowed a lot of spending, but have not provided a strong impetus in getting the club forward. Man United is a huge commercial entity, and the rationale for their ownership seems to be revenue generation more than winning trophies. With this attitude in place, things are evidently deeper than Mourinho's place in the club.