Because mostly only big companies choose their destination. For the rest it's where they live when they found the company. So if more people in Chicago found tech companies you'll get more tech companies. That's the reason SV is a self-feeding machine. People work at one of the many startups or big companies here, get experience, get money, and then part of them start their own companies. That's the circle that drives SV and because it gets stronger each time it's hard for any other city or region to really catch up.