That's largely because the majority of stores in the US lack contactless terminals whereas the UK has plenty of contactless terminals, hell the small corners stores do too.From the US rollout it seems still only a minority of shops accept it. I doubt smaller supermarkets and local shops would accept it straight away, if ever.
----
Those complaining about contactless in the UK, where do you live? London is contactless central and one could, in theory, use Apple Pay with US issued cards in the UK so Apple Pay will be an easy roll out, it's just whether or not banks will be on board.