Not necessarily combined - just that there is more experience in the Bay Area (with larger tech companies there, higher demand for devs, etc-) vs an area that isn't as starved for engineers. Obviously SV vs Atlanta is different than SV vs NYC which is different than SV vs all of the rest of the country.
If you're a company in, say, Atlanta or Boulder, and you need an engineer - there isn't as much competition as there is in the Bay Area. Therefore, it's more likely that you'll find an American engineer at the same price point (which is cheaper [due to legal costs] and less risky than an H1B), so there is little reason to go the extra mile and file an H1B - unless you really need that specific engineer/talent.
Whereas, if you go to the Bay Area, if the demand for engineers outweighs the supply (which I imagine it does), then it's more likely a company will file for an H1B for someone since it's a way of increasing the supply of engineers. The additional capital and legal knowledge (for smaller companies like startups) further spurs the willingness to try for an H1B.
From your response to the other poster - the one thing I would add is that the switching costs for someone on an H1B are higher than those that are not on H1Bs since you always have to have a company sponsor you (this isn't a difficult process, but if you are switching jobs I believe you have to have a company file a petition to sponsor you prior to you leaving your old company). That switching cost is sort of a way of retaining talent (obviously if someone wants to leave, they'll leave - but for those on the fence, they may not bother).