The main reason some Apple application stores have less support from developers, is because there is less money to be made or it's not just suited for a lot of developers.
When you look at Apple's financials, the vast majority of revenue is still tied to the iPhone. Apple's beloved "Services" category is largely iPhone driven, so when you add Services + iPhone, the vast majority of Apple's sales are iPhone related. Developers are going to follow the money. None of Apple's other platforms comes close to the iPhone in terms of cash generating potential for developers.
The iPad is basically just a big screen iPhone with some UI tweaks, so porting an app is relatively easy. Even so, there are plenty of developers that still can't be bothered to release iPad versions of their apps.
The Mac has pretty much been stuck with the same marketshare for ages now. Established developers continue to support it, but we don't see a lot of new Mac apps. Even with all of the work Apple has done to make it easier to bring iOS apps to the Mac, most of the iOS apps I use still don't offer Mac versions.
I don't think Apple quite knew what to do with the Watch in the early days. Is it a platform...or an iPhone accessory? They ultimately landed on iPhone accessory, which makes total sense and further explains why many developers who initially supported it have abandoned their apps. You can't do much on a screen the size of a postage stamp. It's great for quick glances or recording a quick note or replying to a text via dictation, but it's never going to be a platform. There's basically no need for stand-alone, full-featured apps on the Watch.
AppleTV...another platform that simply doesn't sell enough units to merit significant developer interest.
And now we have Vision Pro and the new visionOS platform, a problem in search of a solution. Until/unless it starts to sell significant quantities, there's zero incentive for developers to create apps. As you said, it's all about the money. If you're a developer, the obvious/best place to allocate your time and resources is the iOS/iPhone market. There are no doubt some developers who are very interested in Vision Pro personally and will release apps because of their love and excitement for the new platform, not because it makes sense financially. Once that wave crests, we'll see how well developers support the Vision platform.
I predict that a year from now the Vision Pro App Store will most closely resemble the AppleTV App Store.