It never was all German gold. It was just a very huge chunk of it, stored in a safe place in case the Soviet Union overran Germany. In case of a Soviet attack, Germany was considered lost ground and the first major line of defense was the river Rhine (the Western border of the country), meaning a German government in exile would have lost its gold stored in the country.

I think it’s due to historic risk management - even 50 years ago Germany was far from settled and secured compared to the US economically or politically. Many other countries were similar and didn’t take seriously the seismic risk changes of the past few decades. Now they are.