As a developer you want to reach the largest audience at the fastest speed. For the initial future the Vision Pro isn't going to be doing anything other than running iPad applications in self-contained windows because iPhone and iPad are still where the money is and you gotta eat, right?
WatchOS has confirmed to us that even with a large user base, premium apps do not do well compared to free, ad-supported ones. There is no space for ads on a Watch and so apps have not really taken off.
Now consider the incredibly niche audience of a $3500 headset. Ads popping up is a big no-no so you have to go premium. The market is too small for exclusives so the more popular premium apps will be ones available for cheaper platforms like Beat Saber or Superhot.
Which then begs the question: if the same apps are available on lower cost hardware for the consumer and the Vision Pro market is too niche to build exclusives, is it even worth it to begin with?