Screwing over the international community, as is tradition within Apple.
Or maybe they’re just having a thing at their home and if you’re international and want to come and can afford it then do so.
But putting on full-blown events like WWDC is enormously expensive both in direct dollars and, probably more importantly, people's time. Some big US companies do hold their big user shows (which serve a somewhat different purpose and audience anyway) in both the US and Europe but most just do US only.