The problem is that a U90 grade is extremely difficult to come by and your average carded figure will only get a U80 or U85. It's the rare gems that get U90 or above. So, of course, that means that many collectors will ONLY accept U90 figures for their collections and I've read reports of guys who literally send in 20-30 vintage carded figures in the hopes of getting just a few U90 grades.
Of course, all of those figures are being pulled off their cards to get this U-grade, thereby reducing the number of vintage MOCs. Making it much more difficult for other collectors to find MOCs for their collection and making it extremely difficult for new collectors jumping into the hobby.
In fact, this has been going on so long that there might be carded figure variations that have been completely obliterated from existence and we would never know it because those figures were pulled off of their cards.
It's become such a widespread problem that there is even a website dedicating to educating collectors on the harmful effects of U-grading on our hobby: