This is a big assumptiom by me but I think United have a problem with pulling power as all other clubs of similar size are in bigger cities plus there is the English weather factor.
Throughout recent history, United struggled to get the truly top talent available at the moment. I have been following this forum for ten plus years and can tell you that the posters' wishes almost never materialize.
Then when United somehow manage to bring in proven players from outside England (Di Maria), he wants to leave. All the other signings in the last decade or so were vrought in either from England or from much smaller clubs (ie Herrera).
Self-proclaimed biggest club in the world, United lose a peak Ronaldo to Real after playing their second CL final in two years. At the same time, Real had not made the last eight in seven years at that point. Let that sink in.
Basically, United cannot compete with other clubs of similar size, and then there are smaller clubs with money that can blow United out of the wayer financially (City and PSGs).
So, United cannot compete for the Bales and Alcantaras of the world and therefore have to bring in the Matas and Hereras... At grossly inflated prices, as the sellers know United have money.
United had two spells as an European powerhouse, but those were made possible by once-in-a-lifetime class of 92 and by the perfect storm that was Ronaldo and Rooney developing in the way they have.
Given that United were never really able to bring in top talent and that finding young players first is becoming increasingly difficult nowadays due to improved scouting, the question that arises is if United can ever again becomr one of the best sides in europe.