Perhaps it's as simple as this. AdibokNibro agree to do United a snazzy range of kits for the next three years. They want fifty grand for it, and will charge the club ten quid per shirt. Macron will pay the club fifty grand for the same time period and will charge the club five quid per shirt. Who would you go for?
I don't pretend to know the ins and outs of it, I just suspect that United have gone for the deal that is best financially for them, without the speculation of how many shirts they will sell being dependent on who makes them. After all, it's hardly as if having Adidas shirts will guarantee we sell shit loads more, is it? We might, but then again, we might have roughly the same levels of shirt sales every year regardless of manufacturer.