Short version in above link. 
US (as a region, not USA as a state) has a slightly larger player base than EU and less different support languages. Which means they have different region internal balance.
Playing on EU realms requires an EU account (and subscription / game time above level 20 for BfA). If a Chinese person lies about his / her location and contact details, then that account counts as EU. Also, WoW is available in China, so it is most likely that most Chinese WoW players play Wow there, not here.