As for why the limit(s) were created? My best guess is to that so people wouldn't horde like mad and go crazy on level cap increases. I don't think there was ever a given explanation, and if there was It was overlooked.
Lets say hypothetically that was their logic. Level increases don't happen too often where this sort of scenario would only apply like 1% of a player's experience when playing RoM. A better designer would come up with a way of combating that small use case rather than impacting everyone unnecessarily most of the playing time.
If this indeed was their logic, then they should at a minimum update the MEM limit each level increase as well. The increase would match the same ratio equivalent pulls on the new level vs the prior level due the constant increase in MEM requirements. Example..
(Never changes regardless of level increases)
Amount of MEM gear pulls allowed at max character level: 3
(Current level)
Max level is <x> - Lets just say 90
Amount of mems required for gear at this level: 1190
MEM limit would need to be:
3570
(Next level)
Max level is <y> - Lets just say 93
Amount of mems required for gear at this level: 1300
MEM limit would need to be:
3900
etc..