Because of the Brussels Effect. The EU wanted a single copyright term for their single market, so they picked the member state with the longest term (Germany),

Because of the Brussels Effect. The EU wanted a single copyright term for their single market, so they picked the member state with the longest term (Germany), made that the EU-wide standard, and then put in a reciprocal extension clause basically designed to force America to go life+70. At the time Disney was pushing hard into the European market[0] and having uniform copyright rules between two major trading blocks would be a huge boon to them. Keeping the mouse slightly more under their thumb than it could have been otherwise was just a bonus. [0] Even when it was losing them billions of dollars, such as with the EuroDisney debacle