Why? Who else would come before that? Europe & North America are the most progressive places on Earth for quite a while now.
Its not that its first that is surprising, its that with one exception its only them.
Again: Who else? The rest of the world is in varying degrees of “ass backwards”, especially regarding social issues. Hell, a lot of them aren’t even moving forward but backward.
The thing is that Abrahamic religions were extremely ass backwards on this issue for the longest time. My expectation would be that countries with a majority of atheists or buddhists would never have been as ass backwards about gays in the first place.