Jews were considered beneath the greatness of the composer Wagner or Beethoven because these two composers were German. The previous answer is partially incorrect. Beethoven was born in Bonn Germany baptised on December 17th 1770.
In the Nazi way of thinking, it would be an insult to the dead musicians for Jews to play the music they had composed previously. Also, it further deprived many of the great Jewish musicians from playing music that they loved and enjoyed.
Not being allowed to play a particular composer's music may not seem like much in today's world, but this is a time before television and radio entertainment. People played instruments for entertainment and comfort. Imagine not being allowed to listen to your favorite music.
Most of what was done to the Jews during the Holocaust was done to demoralize and degrade the people who were victims. First, they had to wear a yellow star for identification, then they had to leave their homes and move into a ghetto, after that, they had to leave the ghetto and move into concentration camps where they died in droves. If they didn't die fast enough, they were gassed. In every step along the way to the death chambers, they were robbed of their possessions. Once they arrived at the concentrations, gold filled teeth were removed simply to collect the gold. Upon death, their hair was shaved and used in couch cushions.