Maybe the reason to remove a la carte is because in September 2024 WotC don't want you to buy just your new class/subclass/spell to rebuild your character, they want you to buy the whole PHB. Just my 2cents...
Maybe.
One thing noone mentions: there is an overhead cost of breaking books into parts. Especially with the 2024 books, just buying a subclass gets tricky. Because it might not work in a vacuum. Does it refer to something in the new base class? Does it refer to a new rule element?
So WotC might have to make a decision how to deal with such. Can you only buy a subclass, if you already bought the class? Is this transparent for consumers? Will people with an axe to gind complain that you can't buy a subclass without first buying a class and call it a money grab?
So maybe they decided that this is not worth the hassle and better change the marketplace now, so the rage has gone down already when the new books hit the shelves.
Because, to be honest, every change a company makes causes rage these days in social media.
Lets look at r/brawlstars. So many rage posts after each change... even after the company explains it with data...* how they destroy the game. And then I look around and see more students play brawlstars than ever.
*haters focus on a single nerf to progression and neglect everything they get to compensate for the loss and more). Always thinking all the positive changes for progression could have happened without the negative ones.
Making jokes about the developers, when someone tries to explain how everything balances out. Also neglecting that maybe they are one of the few people with slightly negative impact (because they are the ones who are already far ahead of anyone else...) and they want to deny new people any advantages to keep up. It is a big form of gatekeeping.
Sorry that I got sidetracked... but it is just the same. People often look at things from their perspective. And don't try to take the perspective of other people/companies. So while claiming "changes for no reason" it might be time to step back and try to look at it from a different angle.