Access to VC money.
The money is in California, if you are even in Vancouver the VCs don't want to come to you.
This creates a virtuous spiral where the money is there so the tech people go there, so you start a company there because there are people there, so the VCs go there.
There are very few other centers so concentrated, Cambridge, Berlin, Waterloo are 1% the size.
But eventually some VCs will take the risk of a flight to Chile or Singapore - especially new Indian/Chinese money - and gradually the pre-eminence of silicon valley will decrease
