Likely because hubby has a visa that gets him out of paying US income taxes.
The real question is why the assault on the House of Windsor by US and UK Derp States.
W.T.H.?!
Direct involvement in politics is a no-no, and ESPECIALLY involvement in politics in another country.
The royals might get away with bland statements on global warming or some such nonsense, but direct lobbying of American politicians is another matter altogether.