>Why having MUI as standard equal on all platforms? Because it would make porting easier

The same could be said for ReAction...

>Then you will get t he answer from 95% of the Os4 guys that they abandon MUI because it is not OS4. For the same reason they favor ReAction.

Not at all, ReAction was around and was the official Amiga GUI before OS4 or MOS even existed. There are various reasons to avoid MUI.

>why don’t they take the chance to spread ReAction instead to everywhere

I don't know why. It would be a good idea. If Hyperion are not willing, it might be possible to licence it from H&P or the original developers. Or, of course, an API-compatible reimplementation could be done. It just doesn't make sense to blame Hyperion for the fact that MOS and AROS are missing RA support. Like I said, it was around before OS4 even existed.