The share of Treasuries held by official investors—foreign central bank reserve managers as well as the Fed—has been going down in recent years. But the share of Eurozone bonds held by official investors is soaring, and could now be approaching 50 percent of central government debt. With the ECB holding a rising share of a shrinking stock, there aren't enough German bunds around to meet reserve demand.
The world would be in a better place today if the ECB and BoJ had joined the Fed in quantitative easing early on. Their lag in easing contributed to the policy gap that led to the dollar's large 2014 appreciation.