What does this have to do with translation? I love bashing the US as much as the next guy (after all, so does everyone else in the US when they aren’t being hypernationalist 🙃) but we’re talking about subtitle translation, not conspiracy theorists. From a population perspective, there are more people watching who speak American English. If localization only has the resources to target one dialect, that’s probably the one they’ll go for. Fan-subs are another story (and are usually better anyway)