Because China expects Japan to apologize like this:

This was Willy Brandt German Chancellor back then apologized for the Holocaust. China suffered millions lives casualties under Japanese occupation.

But instead Japanese gives this:

This is Shinzo Abe, Japanese Pm visiting Yakusuni where Japanese War criminals was shrines. Yakusuni Shrine to be fair is not just commemorate war criminals, but why would they shrines war criminals in this shrine? No wonder former Japanese victims were outraged.

The point is Germany has sincerely reformed their way since Nazi Era, their education curriculum ensures everybody is aware the Nazi Crime. While in Japan, the government this to whitewash past crimes.