Guam lives on tourism. About 80% of over 1 millions tourists a year are Japanese. 3.5 to 4 hour flights from Japan make Guam an affordable destination.
At the same time, Guam is also attracting citizens from other Asian countries with visa waiver program.
Besides GUM - HNL, Guam/Japan routes are bread and butter of legacy Continental Micronesia. And with UA hub in NRT, it's picking up connecting traffic from USA as well.