I believe Eliezer was one of the last SIAI employees to move to Berkeley. My guess is SIAI originally moved from Santa Clara to Berkeley because some SIAI employees had rationalist community friends in Berkeley, and when visiting those friends, they noticed they liked Berkeley better than Santa Clara. (I've lived in both places--IMO Santa Clara is dystopian and suburban, but Berkeley is lively and interesting.)

I don't believe there was significant community buildup in Santa Clara before the move. So maybe the takeaway is to make your HQ a place where people want to live for reasons other than just being part of your community?