>I mean, why not? They did so in the past (Windows 10 S).

probably because they don't want to bring up old wounds regarding antitrust. 10 S was trying to go around it by more or less making a mobile device with some desktop functionality. Worked out about as well as Windows 10 mobile.

>but why would it be regulated against now?

well, IOS is being regulated against now, so there's your reason.