[jtheory]

A few possibilities:

- it's simpler for tax/legal purposes for them to hire people who are US residents and legal to work in the US.

- we're in the UK, and hire non-UK-based remote, but we've had a hard time if the timezone is more than a few hours off of London time. In the US, this pretty much restricts you to the Americas (which include a lot more than the US, of course, but this does make a decent chunk of non-US candidates incompatible for them).