California State Route 98 is a fifty-seven-mile State Highway located in Imperial County. California State Route 98 begins at Interstate 8 near Coyote Wells and loops back to Interstate 8 via Calexico. California State Route 98 is one of the original Sign State Routes which were announced during August 1934. The blog cover depicts the beginning of California State Route 98 at US Route 80 near Coyote Wells during 1958.
https://www.gribblenation.org/2023/09/california-state-route-98.html?m=1
CA-98 (as well as CA-94 and CA-96) most likely got its number from being near the Mexican border. I believe this is also one of the most static routes in the state, having not undergone any major changes since its inception (same general location, no freeway alignments, etc.)