There are definitely a lot of factors at play (education, etc. etc.), including a preferential attachment model where cities with established startups and funding opportunities will attract new startups.
You've really hit the nail on the head, however, as that no-compete clause is, while perhaps even enticing to large companies, very restrictive to new startups. We seem to disproportionately have ideas about and go on some entrepreneurial mission in an area we already know intimately, usually related to our current employment. No-compete clauses destroy that entire basket of opportunities.
