In my experience it is a lot of things, in varying amounts depending on the situation (and each of these is both good in some ways and bad in others):

- The USA is a huge and fairly homogenous fairly wealthy fairly risk-positive market.

- The USA has a bigger = better mindset. "Your restaurant is fantastic, you should open another one!".

- Abysmal work-life-balance. Great for making stuff.

- Minimal language and cultural barriers.

- Because of the USA market, you can grow huge before having to expand to other countries.

- Because you can grow huge in the USA market, that is where the funding goes.

- All the the USA's allies didn't mind too much, because they are(/were?) allies

- All the non-USA-allies developed their own markets (Russia, East Asia, Africa, India, etc).

- And so now, given the messaging coming out of the USA, EU governments/companies/people are giving their USA-entanglement a second look.

Which, being in the EU, is pretty exciting. I feel like there is a lot of opportunity here right now.

Disclaimer: I'm sure there are 1001 reasons, and the above is just one potential view.