Nobody really actively chooses Apple Mail.

It's just that they buy iPhones, and they want a total no-brainer, like, a phone that's fully set up and ready to use without them having to do anything because it, like, totally confuzzles them 'n stuff. So whichever friendly salesperson sells them their phone also sets everything up for them. Including an e-mail account because they need one for their Apple account, but they don't know if they've got one.

If they buy an Android phone, it's the same, only that they get a Gmail account if they don't happen to already have one.