The only member that got development was Axel
While I agree Axel was heavily favored for character development, he was not the only one to get it. I will go into detail below.
or does SE have a contract with Disney that they absolutely have to retell the movie's story while in the respective world?
In some cases, yes, it has been stated that Disney contracts somewhat "forced" the plot of the world to be driven in a certain way. Especially with the Frozen world which refused to let Elsa do anything considered "villainous" and neither princess was allowed to be a party member (among many other unstated things).
I somewhat understand your frustration--I sometimes would also wish for more cut scenes with some of the "villains" because they are interesting and fun. However, there is a sizeable chunk of people who also enjoy fighting Disney bosses since the Disney experience is a big part of what KH has always been.
So, while I don't completely disagree with you that some of the KH characters have very shallow development in their initial debut game, I don't fully agree that they never get development or background information. First of all, while the org 13 were all "antagonists" at introduction, they were definitely not all created equal on that playing field. Some were definitely meant to be secondary characters and others tertiary at best. However, over the course of the series as a whole we have actually come to learn a decent amount about most of the original org 13, honestly. Most of them have even had their own mini character arcs (Zexion, Vexen, Saix, Axel, Roxas, Xion, and Xemnas in particular) while others are clearly building up to having major relevance in the next saga (Xigbar, Marluxia and Larxene with Demyx, Saix and Luxord as a strong possibility).
Kingdom Hearts as a whole tends to stretch its plot across a very, very long time and it feels like Nomura likes purposefully leaving certain areas of character's stories blank so he has a slate to fall back on later when he needs a character to fill a new/interesting role. If Nomura told us all the backstories of all the org members right off the bat, he would lock those characters into a defined position which would be difficult to change later. I think Nomura enjoys having his cast of characters be a bit more flexible since he seems to enjoy having them pop up in unexpected places. Whether one considers this good storytelling or not is up to debate, but it's why character development for KH characters has been generally splotchy and in chunks, unfairly distributed around whoever is "relevant" at the time.
So yeah, looking at the series as a whole, we absolutely have had character development for quite a number of the original Org 13 beyond just Axel.
You mentioned Zexion as a "flat" character, but KH3, specifically, actually did a really neat job of fleshing out Zexion(Ienzo) and Vexen(Even) in particular, imo. They were previously very forgettable evilish dudes in CoM. They died, and were never really mentioned again. In fact, both of their deaths were more used as character development tools for Axel specifically, since their deaths were showcasing Axel's ruthlessness in achieving what goal he had set in mind. All-in-all back in those days Zexion and Vexen were basically kinda
there. However, come KH3 we actually see into their motivations quite a lot. Their personalities got to shine as decent tertiary characters. We understand why each of them were acting as they did. Ienzo was young, felt bitter and betrayed by his father figure, made a bunch of rash decisions while lashing out as young kids tend to do, and actually ended up ultimately getting a reconciliation moment with his parental figure to boot. It was a very nice mini character arc that was resolved. Vexen similarly was also looking to resolve his own inner guilt and it was a pretty cool twist when it was revealed he was the one who saved Ansem. I was fully expecting Saix to have gone rogue, but Vexen was a "whoa...okay then!" moment, imo. Looking back, makes a lot of sense. I think it was well done and it was really all the character development those two actually needed because they aren't really
that important to the story. But now that they have solidly established themselves as "the good guys' geek squad" they can sort of pop in now and again with some new deus-ex tech that they have been "developing" in the background.
As for Saix--BBS and KH3 gave us more introspection into his character/motivation so he wasn't ignored either. Considering he was even accepted into the main character squad in the final cutscene of kh3 means we are probably not even close to seeing the end of Isa. The question of mystery girl X, a plotline he is directly involved with, is also very much up in the air. We learned about how sad and jealous he was, how that twisted his motivations and cost him quite a bit. We learned about his childhood, about why he was doing what he was doing the whole time and why he fell down the wrong path.
Larxene and Marluxia had their backstories revealed and developed in UX, which was even hinted at slightly in KH3 (even though we already knew what that hint was referring to at the time). The two of them, being known keyblade wielders in the past, are no doubt going to be VERY important moving forward. Also it is nice to have a character with an established sibling relationship, its a breath of fresh air in the series. It is also unclear exactly how important Strelizia is going to be. Possibly quite a lot.
Xemnas obviously had a lot of development over the course of the series if you consider Xehanort as a whole. It was incredibly interesting after the final fight with him in KH3 that I actually felt
bad for him when he died. I HATED that guy to pieces in KH2 then suddenly he revealed how just...empty and meaningless things were to him and it was kind of...ouch. Really showed how depraved Xehanort's actions ended up being in that not only were his plans ruining the lives of others, but he was even ruining his own life--making own iterations of himself miserable to accomplish his goal. Xehanort was definitely...thorough.
Xigbar had a TON of information revealed about him over the course of several games. First with the hints of him knowing about "other wielders" in KH2, to him knowing about Ven in Days, to his inclusion as Braig in BBS, to the ultimate Luxu reveal Xigbar has been a constant mystery card of how much he actually knows.
Luxord has kept pretty quietly in the background, but now with the mystery "driver" from Quadratum looking like him it raises a ton of questions that possibly are going to be relevant very soon.
Nomura has hinted something about Demyx, we just haven't seen that payoff yet.
The only real org members so far who have been completely shafted in terms of information are Lexaeus(Aeleus) and Xaldin (Dilan). Poor dudes got absolutely screwed in kh3. So...I guess 2/14 members of Org 13 are pretty bland and pointless so far? Honestly not that bad.
So, while your point stands that perhaps if you isolate each game individually and look at the content that's in there...there isn't too much. When we are first introduced to the Org they are incredibly mysterious and we know next to nothing about them (which I actually really like, I think it was really fun having such an unknown group as antagonists at first). However, because KH's story has been one long continuously unfolding series of events the development has been coming in at a slow drip speed pace. That doesn't mean its not there, it's just taking its time.
Again, is it the best way to tell a story? Eh...debatable. But it makes sense considering the way KH story is. The characters come out of the gate with very strong, definitive personalities and we slowly learn more about them as time goes on once their background information becomes relevant or their role in the story changes. If it isn't relevant, it's mostly not mentioned. This is why despite being the main character we know basically nothing about Sora's parents apart from one or two lines because it's simply not necessary...yet. But you can bet if Nomura decided to have Sora's dad/mom be randomly some important figure we would suddenly be learning all about his family.