An obvious reason that the snowy owls are not eating California Ground Squirrels is that ground squirrels hibernate during the winter.

My reading of the California Ground Squirrel account in Land Mammals of Oregon (Verts and Carraway 1998) suggests that the specifics of hibernaculum in this species are under-studied along the coastal strip. Any statement about the presence or absence of ground squirrels in coastal Oregon during the putative hibernaculum elsewhere in the species range probably deserves one or two caveats about what is actually known....