2. The probability of Unicornism is directly related to the size of the market you can access. US is the biggest single market on the planet.
3. Accessibility of markets: While EU is the second largest single market [0] US companies have entered european markets just fine. On the other hand, to operate in china, it's best to be a chinese company. Hence, to fund growth on a global scale going native in china is an obvious move.
[0] https://en.wikipedia.org/w/index.php?title=List_of_countries...