There are dozens of answers here that explain why institutions buy sovereign debt, in general.
What those comments don't explain is why anyone would buy this particular sovereign debt.
So: why would anyone buy negative-interest-rate German bonds when U.S. Treasury bonds still have positive interest rates, and are available in much higher volumes?
Because those positive yields are only available if you don't hedge your FX risk. Most institutional investors have a mandate to hedge FX risk and this will take UST returns for EUR investors negative.
Because you’re a European and have to pay your taxes (or your investors, or other people) in euros, so don’t want any exposure to the Euro/Dollar exchange rate.
What those comments don't explain is why anyone would buy this particular sovereign debt.
So: why would anyone buy negative-interest-rate German bonds when U.S. Treasury bonds still have positive interest rates, and are available in much higher volumes?