I suspect UK was left off because POTUS has resorts in Ireland and Scotland, and no where in Europe. There's nothing credible about UK being off the list while every surrounding country is on it.
Not that it’s the same thing, but being an island (so no walking traffic) and having some screening in place means that the UK simply doesn’t have certain diseases (like rabies) that are at least somewhat common in Europe.
Honestly though, the UK is one of, if not THE, staunchest ally of the US and banning its citizens would cause some minor political issues.
Having travelled back to the UK from a Schengen country in the past few days, I can also confirm that there wasn’t so much as a poster in the airport, let alone any actual screening.
I don't think I've ever had anything that even counts as token screening flying into the UK from Europe - passport check is automated and I've never even seen someone at customs for many years.