Here's the rub, at least for the time being, the US Tbill is still considered to be relatively high quality, low risk. When bad things economic happen elsewhere the money flows to US Tbills and it's considered a "flight to quality".
I'm not saying that this is how it should be, I'm just stating the way it is.
As bad as our federal government's fiscal practices are, and I completely agree they are bad, the perception is it's still among the safest place in the world to park money and expect to get paid back without losing your shirt via devalued currency or unpaid debt.