SF is a startup hub because it happened to get a little traction from a few early companies, and then network and scale effects helped it take off. Same reason Detroit was once a big auto hub, China is today a big manufacturing hub, Facebook the biggest social network, etc.

Once a regional agglomeration like this is established, it's hard for someone else to match (just like it's hard to set up a new Facebook).

And, also, once such a hub is established, people of course look for reasons to explain why it won. Mostly they're hagiographic "just-so" stories, of no real benefit to anyone looking to replicate it.