...Huh. I wonder what the problem is with Aladdin? Seeing as all of the movies are on there, I have no idea what the problem would be with the series. Looking around, it seems that no one has a concrete answer. Honestly a bit bizarre!
No one knows why stuff like The House of Mouse isn't on Disney+ either... it just isn't. The 101 Dalmatians series wasn't on Disney+ when it launched, but was added around a year later. I can't see how it would have been a problem to have it there from day 1. Clearly Disney has internal reasons for not (currently) including this or that old show on the service, but what they are is not easy to discern.
I remember hearing once that House of Mouse was actually a nightmare to make. I know that Disney owns all of the characters, but I could actually see this being the case. Disney Infinity was in a similar position; despite Disney owning all of the characters, there were very strict rules about how certain characters could be depicted, how certain characters could interact with others... it made for a very difficult production with a lot of limitations. I remember one of the heads of the team was pushing very hard for certain characters, but Disney wouldn't allow them to be used. I could be wrong, but I think Kingdom Hearts faces similar difficulties. Maybe there's something similar with House of Mouse? Like, maybe they've changed their mind about how Mickey was portrayed or something? Or maybe some characters are in a position where footage featuring them goes against a current rule of Disney's? I dunno, I'm just guessing too, but I could see that being the case.
It's like you say, I suppose; Disney have their reasons, and we'll probably never be privy to them. They make decisions that are downright confusing, which can be frustrating, but unless there's a huge demand, it probably won't happen unless they decide to do so themselves.
I have no idea why, but ducks just tend to be the most compelling characters.
Donald, Scrooge, Darkwing, Howard, Daffy, Duckman... the list keeps growing!