1. Investors like to talk in person. They don't feel like the have the soul of a company until they've shook hands with the founders and had a few conversations. There is an old truism that being within 30 minute drive of an investor is the best indicator of whether they will fund your company.
2. Nerds and entrepreneurs like to talk. A lot. While HN is a lovely way to talk, it's also pretty low bandwidth and it has high latency. Talking to others is how you figure out your business, and in the rest of the world, talking about a startup "wastes your time and annoys the pig".
3. It's reasonably nice to live here, so a lot of smart people live here by choice. You have to hire smart people eventually. And not just nerds. You need lawyers who know about startups and will work on retainer. You need accountants who understand that you're an idiot about taxes and finance and need guidance. Basically, the friction of doing business is lower in the valley (SF isn't really the center of this magical place, though it's gaining lately...the center of the action is Palo Alto and surrounding regions).
4. Stanford. Stanford is designed to churn out technology startups. It has a great MBA program and a great comp sci program, and it is extremely involved in startup culture. I doubt any other university is as tuned to producing students who start startups as Stanford is.
There's probably other stuff, but I think those are the biggies...and the ones that can't be easily replicated by mere talent or ambition or stubbornness. Luck might do it. But realize that even lucky companies (like Boston-based and YC-backed Tsumobi) end up coming to the valley to get funded. Despite being in Boston, the second largest "startup hub", for a year since WFP07, they didn't get further funding until they came out to the valley to stay for a month...and Ron Conway spotted the raw intelligence they bring to the table.
The benefits of the valley are obvious after you spend some time here. Will it change? Maybe. But not fast enough for it to make a difference to your current company. It'll be decades, possibly more, before tech startups outside of the valley have the same odds as those in the valley.
http://news.ycombinator.com/item?id=182746 - Why You Must Be in Silicon Valley
http://news.ycombinator.com/item?id=174298 - Ask YC: Where to startup? (If not the US, then Canada or UK?)
http://news.ycombinator.com/item?id=116600 - Why Seattle might be a better place to start a company than Silicon Valley