> Relatively nobody in the US cares about soccer, much less travels for it.

Utahns travel to US states with World Cup games for reasons other than the World Cup. Its Summer in the Northern Hemisphere. Utahns do travel, for example, to the San Francisco Bay Area for graduation ceremonies, to visit family, and for sightseeing. These Utahns may mingle with foreign visitors.

I’m not making a case that transmission of measles from a Utahn to a foreign visitor is likely, but the failure to consider possible routes of transmission is exactly why vaccination is so important.