I agree with the high quality staff issue (it's still Silicon Valley and some people remember how to do high-tech manufacturing) but I think they also wanted to

I agree with the high quality staff issue (it's still Silicon Valley and some people remember how to do high-tech manufacturing) but I think they also wanted to be close to potential customers (rich companies that use tons of power and are flush with cash). Not to make deliveries to customers easier, but to make it easier for people to visit them and say "ooh, solar cell factory!". Plus their VCs probably didn't want to drive to Sacramento