Really, INME, just “in the cities” (with maybe also the coastal mountain areas between the coastal cities). The inland rural south is no different than the inland rural north in this respect, AFAICT.
But the population is largely in the cities.
Yes the incredibly empty "Empty Quarter" of the state is large, and empty. As in nobody lives there. The best-selling vehicle in Modoc County may or may not be a truck, but the fact is of no interest.
I'd be cautious about blaming it all on fuel prices. I'd argue that the increased wealth of California residents relative to the rest of the country also plays a part in why a vehicle that is too expensive for most people made it so high on that list in California.
Tesla Model 3: $46,990[1]
[0]https://www.ford.com/trucks/f150/models/?intcmp=vhp-attribut...
https://www.energy.gov/eere/vehicles/articles/fotw-1168-janu...
[1] https://www.kbb.com/reviews/pricing-your-next-ford-f-150-it-...
I would argue it's more because auto manufacturers wish to streamline their production lines. Body-on-frame is very flexible and allows them to re-use most components across different products.
The marketing departments then produce Television ads questioning your masculinity if you don't drive a vehicle based on a commercial truck frame. Suddenly "it's what the customer demands".
I'm not sure what the prices and fees are in California, but for comparison, in Mississippi it costs me about $45/year for registration and today I paid $4.17 for fuel. If I recall correctly, vehicles newer than 20 years old can cost significantly more to register here, but I don't have anything that new.