The main reason: it just is no business. There is no money to be earned - otherwise some chinese company would do it. But even in low cost countries professional developers cost more than the revenue of such an app.
So apps are either developed by the drone vendor and paid with the drone or done by hobbyists. I bet litchi was borne as a hobby project.
Obviously no Apple hobby developer has had the need to build an app for
Tello. The Ryze app seems to be better on IOS, at least much better video quality.
Also one reason is that many (not all! ) apple developers are more of the "hey, sexy UI and cool sounds!" type. Generally more graphics oriented folks than true software engineers. These folks would have some trouble doing the low level bit & byte protocol analysis required to build an app like TelloFpv.
Developing such an app for IOS would be easier than for Android because the market is not fragmented with hundreds of Android versions and thousands of phone models. But its just not my thing, I am not interested in Apple, I don't have the time, I don't feel like spending $2-3k just for the initial setup (mac book, a couple of use iPhones, iPads, etc).
Keep in mind:
Tello is already pretty old, and may be discontinued any time. I doubt it will do another year, which leaves very little time for an IOS version to pay for itself (after 3-4 months dev time).