Also, central bank QE has vacuumed up highly-rated sovereigns, creating shortages among private institutional and individual buyers. The Fed stopped buying securities in late 2014, but the European Central Bank and the Bank of Japan—which already owns 34% of outstanding Japanese government securities are plunging ahead. The resulting shortages of sovereigns abroad and the declining interest rates drive foreign investors to US Treasury's.
Also, as we’ve pointed out repeatedly over the past two years, low as Treasury yields are, they’re higher than almost all other developed country sovereigns, some of which are negative. So an overseas investor can get a better return in Treasury's than his own sovereigns. And if the dollar continues to rise against his home country currency, he gets a currency translation gain to boot.