Probably because 5W-20s have been widely available in the US since both Honda and Ford adopted them a dozen or so years ago.
Probably also because the recommended OCIs are much longer in the UK than they are in the US, mainly because oil is so much more expensive in Merry Old than it is here.
There is no way that Hyundai would be able to shift any cars anywhere in the EU if they tried the 3750 mile OCI in "severe service" with almost any use of the car being "severe service" nonsense they play here.
They have to recommend much longer drain intervals in EU markets, so they've recommended a grade better able to withstand the shearing and fuel dilution that accompany longer drain intervals.
People like to claim that CAFE has driven US oil grade recommendations.
It's equally valid to maintain that the need for much longer OCIs has driven grade recommendations in other markets.
We use thinner grades here because we can afford shorter OCIs.
Motor oil here is cheap by the standards of the rest of the developed world, as are cars, housing, food and just about everything else.
If we had EU level motor oil prices, we'd be using thicker grades on much longer drain intervals as well.
Dave Newton would be smiling with approval.