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As for iOS-only I’d assume lack of resources.

Developing and maintaining an app on multiple platforms (and keeping the code bases in sync) is no small undertaking (hence the existence of write-once / hybrid app frameworks such as Ionic).

Regarding iOS 14 my guess is SwiftUI.

In particular, why not a webpage? This doesn't need to be an app.

Probably a side project for a full time iOS dev who wanted an excuse to mess about with SwiftUI and decided to use the tool they are most familiar with

I’m testing a bunch of SwiftUI2 features right now so it’s constrained to iOS 14 for them. If you’re on the beta, you’ll get it when iOS 14 is released.