There are issues when running on macOS (eg crashes when trying to play audio), and given that there’s an existing desktop version available, I’m afraid it’s hard to justify the time required to fix these issues and ensure future ones don’t crop up.
Ah, so you already tested running AnkiMobile on macOS? I wonder if a justification could be that ongoing maintenance of the iOS version on macOS would prove to be easier/smoother (once you iron out the existing creases), than getting the existing desktop app to cross-compile.
Maybe there’s a way to allow beta testing of AnkiMobile on macOS in meantime, perhaps using TestFlight? I can’t remember if AnkiMobile is open source/donorware.
Replacing the macOS version of Anki with AnkiMobile is not an option I’m afraid. Aside from the fact that it would require all macOS users to purchase it, AnkiMobile currently lacks various features the desktop version has, and it is unable to run add-ons that some users rely on.