He was appointed by George W Bush, a Republican of course.
Usually, not always but most of the time, presidents appoint judges according to their party affiliation.

While a number of lawsuits have been filed over Trump's action, the Washington state lawsuit was the first to test the broad constitutionality of the executive order. Judge James Robart, a George W. Bush appointee, explicitly made his ruling apply across the country, while other judges facing similar cases have so far issued orders concerning only specific individuals.