When it comes to export controls and sanctioned entities it doesn't really matter what Red Hat would like to do - they have to comply with the law in the jurisdictions they work in. Even if it was purely a community project individual contributors face a similar liability if based in those jurisdictions.
When it comes to sanction lists there is a fair amount of commonalty between the US and Europe. This is really something to complain to government about.
Was it explained? There was a comment that Klingons don't like to talk about it but no explanation I remember.
My personal head canon was something went wrong during the DISCO timeline and somehow the process that gave us Tyler ends up infecting the rest of the Klingons who spend the next 50 or so years as swarthy humans before eventually being restored by scientists to the TNG/TOS films ridges we all know and love.