I think it's mostly culture. Compared to other regions, US favors innovation/risk taking and sees software developers as value creators rather than cost centers. There's also the fact that USD is the global reserve currency which makes it much easier to raise capital.
> USD is the global reserve currency which makes it much easier to raise capital. Could you explain more?
1. There's more people around the world with USD who are interested in putting it to work.
2. The US government can easily print and borrow money denominated in USD, which leads to more USD floating around, which ends up in the hands of people from #1.