I would suggest, just based on my intuition and no data (if you want to provide the data you're referencing, that'd be great), that this may be conflating correlation with causation.
Successful web startups are based in SV because to be a successful web startup, you need LOTS of money to burn on marketing and promotion. To get lots of money to burn, you must get investors with the appetite for watching their money burn up. Those investors have built out SV as their nexus not due to any magical qualities of the place, but primarily for personal convenience (traveling sucks, etc.).
The VCs in not-SV do not allow startups to run anywhere near the same way as the SV guys do, despite their attempts to imitate the YC model (most of which are, IMO, outright fleeces on naive locals). Non-SV VCs are just used to doing much more traditional deals, not seed-stage tech financing.
The people who want to start VC-backed companies thus have to trek out to SV, and then once they're established, they stick around because that's convenient for them too.
If you can decouple from the VC ecosystem, or if you can find good local VCs that know the field (and you probably can't), there is every reason to NOT start in SV.