Somewhere someone had proposed a new rule that would require all parking structures to be built on level floors, with the idea of allowing a future conversion when the conditions are right. Slanted floors negate this option from the start.
The additional work required to build a level floor over a slanted slab seems minor when compared to the total work involved in converting a garage to e.g. residential use.
A (pipe-)dream of mine is that parking structures will eventually be taken over by tiny homes...