This is because of the Dodd-Frank Act passed by US congress in 2010.

This regulation also applies to all the other foreign brokerage firms, that CFTC does not allow them to let US residents to open accounts.

It is the same condition for any other online brokers outside the US.

If a foreign broker (outside of US) accepts US citizens, you are recommended to check the license of the broker to see if the broker is authorized to do that.