Cant answer for their reasons, but anecdotally I've seen this happen a few times and in each case the 'founder' of the app already had a load of experience developing for iOS / macOS / Swift. They then hired someone / learned how to do Android once the prototype was complete.

A lot of these things start out as ideas or hobby projects too, so generally the app starts being built for iOS because why wouldn't you write it with the stack you're already comfortable with / can use your own phone to test it.

I don't necessarily think its always a conscious 'we must do iOS first', but if you own an iPhone already and do iOS dev at work...