It has to do with the loading of these items. Stationary items that are not customizable contain far less data to load. Then interactable items contain even more information. Finally, special items such as flower pots, npcs, mannequins, and aquariums take far more information. For the longest time while there was no official cap on interactable items there was a set limit to how many interactable items such as food, certain lights, showers, and other such items could load at a time. If you had more than that in a given area only some of them would load and a lot of items would vanish from sight until you were right on top of them.
It took years for them to be able to work around that, and this was even after the initial item limit increases for housing. Aquariums and other items like that require FAR more information to load, and so by allowing even more of them you'd end up having rendering issues just like used to be caused by interactable items. And I'm not even talking a few items disappearing, I am talking like half of a floor vanishing from sight in some cases with the old thing. With these items it could be even worse. Now it could be worth asking them to do as kaynide suggested and making it a cap for all special items like that and allowing people to chose. The only issue I could foresee is that each of these different items have their own unique data load, which is likely why they each have their own limits balanced within the limits of others to prevent issues.
And just a quick edit. The reason why Apartments and FC rooms have a lower limit is because they are all one floor. The game only has to render a single floor at a time for each player, so the moment we're out of sight of the first floor it no longer needs to render and can be made dormant.
