Culture is very important, but keep in mind that USA is the biggest market without massive barriers like in EU (language, regulations etc). Look at B2B.
In the end the equation may consist of: culture, weather, big markets, limited regulation (non-compete not enforceable), free enterprise, proximity to certain institutions (Bell labs, NASA, Stanford), rich people (angel investors), hackers
In the end the equation may consist of: culture, weather, big markets, limited regulation (non-compete not enforceable), free enterprise, proximity to certain institutions (Bell labs, NASA, Stanford), rich people (angel investors), hackers