I think one of the key factors is the availability of funding.
What Else?
Surely, funding is very important and existing startup ecosystem (meetups, incubators etc) is crucial as well. But since we at Teleport are helping smart people move, we are also hearing from them what are the most important issues for them. This is the top4 and it has some surprising elements:
1. Low pollution 2. Low living costs 3. Low crime 4. Tolerance
While low costs and crime are very understandable, pollution and tolerance could be a bit surprising. But that shows that startups want to grow in cities where their employees can live a good life - it's safe, it's green and the level of tolerance is high. So focus needs to be much wider than just the capital.
Different cities have very different challenges, here is the comparison of London and Berlin: https://teleport.org/compare/berlin-and-london
While the level of startups and capital is surely higher in London (the best in Europe), the environment and living cost are better in Berlin - every city must find their own marketing story.
The reason this works in Silicon Valley there is a critical mass of companies (and hence deals) financially structured to "go big or go bust." Hence a portfolio can be diversified. It is unlikely that a random city will have sufficient companies built this way to support an investor community optimized for startup investing.