It's true that Shockley picked Palo Alto because of his mother, but it would be unfair to not also credit Stanford who had an outsized role in helping solidify the Bay Area as the hub for "Silicon Valley".
Many many many companies tried to move the industry away from California since the 50s, and every time, those companies star players ended up relocating to the bay area.
California's free-wheeling culture permeates at all levels, from investors all the way down to the individual contributors. It just wasn't an accident that Silicon Valley happened the way it did.
Wine, Movies, Farming, Banking, Education, Tech.
California is just a state with a long history with a weird mixture of entrepreneurs you won't find elsewhere.
Only seemingly non-random factor you gave is the Emerald Triangle, which could be the answer.
As to why CA, I don't think there is any one reason. The weather and geography are huge. It was a lightly populated state in the 1950s, so real estate was much cheaper than on the heavily populated east coast (big for both start ups and 'artistic' types). And the gold rush mentality is/was pervasive, which I personally think leads to a government and culture that is generally supportive of start ups and small businesses. While CA has a reputation for being unfriendly to businesses, the taxes and regulations are largely targeting big businesses. So that leaves more room for small start ups to compete.
Anyways, people are always leaving the Bay Area. If you came here five years ago, first you were new here and second, you've known people who have left.
So Keith Rabois left for Miami. Great for him. The question is whether the next Keith Rabois will spring forth from Miami. Unlikely.
A much more reasonable interpretation is that Shockley was going succeed no matter where he went, not that Silicon Valley is uniquely situated, especially with the dominance of east coast companies in defense contracting.
https://steveblank.com/secret-history/
Ames Research Center dates to 1939.