It might also have something to do with that fact that prior to Pep signing for City, both Manchester clubs were underperforming.
Guardiola was at Bayern (i.e. gettable for anyone that wanted him) and Mourinho was not on a job.
The United board must have realised for some time that van Gaal wasn't going to work out, so in theory, there was nothing stopping United from making a big decison to court Pep, get his directors in place and sign him.
We didn't. We did nothing. If we had done something then Pep would have joined us; of that I am certain. Why would any manager choose City over United, all else being the same (that's rhetorical - the answer is that they wouldn't)?
City swooped in and snatched him.
United signed Mourinho as a counter. Would we have signed Mourinho if City had instead signed, say, Ancelotti? Probably not. I think that our board wanted to go down the "look what the other side have done... we better go like for like" path.
Now look at our general football and then look City's general football. I would hazard a guess that trophy count a season ago is pretty irrelevant when we look at the big picture, which is that United look nowhere near City's level in terms of football and to an extent ambition.
That is probably why a lot of our fans are currently not completely happy.