Money.

Apple gets 30% off your app purchase and/or in-app purchases, while it gets 30% from your subscription for the first year and 15% for the subsequent years.

So, if you think your app is worth 10$, you lose 3$ and the ability to make more money and to support its development (if you do, they have to be in-app purchases, like extensions, etc. and again 30% is gone).

On the other hand, if you sell it with a 0.83$/month subscription (= 10$ / 12months, just to be "fair"), the first year you lose 3$ but you gain later 1.5$. And btw, I don't even think 0.83$ is allowed (AFAIK the price points start with 0.10$ so you can't have weird numbers like 0.23$ or things like that), so you round it up to 0.89$/month or 0.99$/month and you gain even more money, which will be used also to keep the development going.