The US has proven quite willing to just take assets off Russians for political reasons. Fitch probably isn't going to say it outright; but if a country has a record of saying "nah, I'm taking it back" about someone else's assets then it seems reasonable to rate them below an AAA.

Even if the US doesn't fail, it isn't really credible to claim its debt is an AAA security. For a lot of participants in the market there is an obvious risk that the US will take their money and not return it.