I think of Silicon Valley as on the economic center-right, but socially liberal. Lots of techno-libertarian rhetoric (I find myself well to the left of the average techie on that), but cultural-conservative rhetoric doesn't play well. Candidates who are anti-gay, anti-abortion, or playing up the importance of religion in public life don't play well. The GOP has had a strong element of that cultural-conservatism for a while now, strongly damaging its brand among some demographics. The Democrats end up as the default option more than what people strongly support (I know many more people who were fired up against the GOP rather than for the Democrats in the last election).

Imo, the Democrats are a pretty miquetoast centrist party on economics, hardly "far left". They're broadly in favor of free trade with a modest safety net (quite modest, by first-world standards). Heck, I'm not sure more than a smallish minority of the party would even qualify as true social democrats, let alone actual leftists.

On policy terms, I would disagree that tax rates are strongly relevant to a startup scene (not irrelevant, but not one of the more sensitive environmental factors). I do think regulation is an important aspect. One reason I would support discontinuing the use of regulations to achieve quasi-safety-nets and replacing them with direct safety nets. But as for why SV is in SV? I think culture, networks, and education have a bigger role in it. Otherwise you'd expect a vibrant startup scene in lower-tax parts of the U.S. than California, Massachusetts, and New York, and we aren't really seeing that.