I don't think that's true. If I wanted to put it colourfully, I would say that anti-realism was born in Britain and died in America. Mackie, the prime figure in moral error theory, was an Oxford philosopher. So was Ayer. Largely stemming from Ayer, the non-cognitivist tradition dominated British ethical theory for decades. It went very well with the linguistic turn in anglosphere philosophy at the time. It still has adherents, the most significant of which is probably Simon Blackburn (a Cambridge philosopher). In contrast, the most trenchant statements of moral realism seem to have arisen in America with the the last two or three decades. David Brink is a philosopher at the Uniiversity of California and Michael Smith comes is a Princeton philosopher. I can't think of any particularly compelling anti-realist figures in America (I am sure this is a fault of memory). At the least, it is my impression that moral realism is at least as popular in the US as it is in Britain. .