The U.S. Interstate system was designed for 70mph. California's freeway system is older. IIRC, California built the first "freeway" in the U.S.[1] It wasn't until I moved to California that I ever even heard the term, freeway, used regularly; elsewhere in the U.S. most people use the term, highway, as a general word for a fast artery. Some east coast locales often have their own terminology (e.g. turnpike) for the same reason California does--they were the first to build such infrastructure, and to do so long before the interstate system.

FWIW, the U.S. Highway system predates the U.S. Interstate system, and is still primarily composed of a patchwork of pre-existing state roads, "freeways", etc. At the time the interstate system was created highways weren't limited access--and still aren't necessarily, though most people don't understand or obey the technical distinctions in the terminology, or what type of road they're driving on.

[1] See https://en.wikipedia.org/wiki/Arroyo_Seco_Parkway. It was designed for 45mph.