It's hard to replicate SV's investor network. VCs in other cities definitely form small communities, but everyone has ties to the valley. If you're in the valley, there is also a critical mass of other companies to provide services -- many VCs use the other companies in their portfolios dogfood each other's products, which helps create networks between founders and engineers as well.

There is definitely a critical mass in SV that's really hard to replicate elsewhere. Not that it can't be done; but it's a lot easier to acquire funding and talent in SF than elsewhere. And I say this as someone whose focus is outside the SF area.