A dense cluster of well-moneyed nations with similar sporting histories leads European Championships of many kinds to be a major regional competition that Europe’s national federations and media emphasize as a chance to earn (what are in their mind, only slightly less major than global) championships and medals. (This is true of XC as well as on the track in Athletics—it’s also true in other sports, e.g., soccer.)
Also, being club-based, many endurance sports’ training methodologies are a bit more old school, and allow for some “fun” racing in a way that the professionalized, scientific training of son US groups doesn’t.
Lastly, there are actually XC competition opportunities throughout Europe, and those only exist in pockets in the US and are of variable competitive quality. Big travel for a non-competitive off-season race doesn’t make sense.
(Oh, and one more thing—travel to World XC is usually easier from Europe, and that makes it more worth it to prioritize qualifiers.)