Well, I'd say it's the following, based on what I've read and heard:
1. Successful companies in the area, especially ones which started as venture backed startups or pay really well. Many new startups are founded by people who got paid a lot of money in their tech company careers, and many of them seem to be backed by entrepreneurs who made it big in the past.
Also shows those wanting to start one that there's success to be had in such a venture.
2. Lots of 'easy' money going around for new companies. This is one major reason it's much easier to do well with a startup in the US (especially Silicon Valley) compared to Europe; the former has people willing to spend millions on a moonshot that's unlikely to go anywhere. The latter won't back it, and will miss out on the next Google/Facebook/Uber/Netflix/whatever because it's 'too risky'.
Also, when I mean lots of money, I mean lots of it too. Note how much your average US startup seems to get from VCs, then compare to how much your average European one gets. You need a lot of cash to get the best engineers/designers/whatever involved. $50,000 funding probably won't cut it.
3. Access to good education nearby. Not quite nerds as Paul Graham says, since I feel it's more those in university who are best placed to get involved in startups. They've got a lot of time, no real commitments, a bunch of other smart people nearby in a similar situation, and enough oddball ideas to turn into businesses. Actually, when I think about it, universities seem like the best place/time for a lot of young people to get involved in a lot of crazy ideas for the same reason; note how many YouTubers and internet celebs got started in college too.
Oh, and maybe a cultural attitude that treats being an entrepreneur as something to be respected, and building a company as a good life goal to have. Way too many places still have this idea of 'success' as 'get a job, work up through the ranks, get married with two kids and a dog and live in the suburbs somewhere'