Actually its driven by the need for localized labor and unfortunately due to high housing costs, labor can't afford to live in downtown SF/LA and so a lot of time is wasted getting to the job site from the outer suburbs.
Ok true. Just would add that high housing costs are actually the sum of asinine zoning laws + high labor costs and that high labor costs are due to high housing costs. :)
And housing costs are high because of asinine zoning laws, as OP mentioned.