They explained why already. They like to build campaign around an immersive character or group of characters. The problem with the German faction is how can you make a cool and nice character belonging to the Wermarch.
There are many games were you can make the bad guys looking cool, but it is a really risky task when your bad guys are based on the Nazi. This could be assimilated to Nazi propaganda, revisionism etc...
Not to mention the potential interdition to sale in Germany and some others European countries.
such bullshit, you can take a famous wehrmach commander like Rommel who was one of the best panzer commander and in the same time tried to assasinate hitler.
no one is going to say that rommel was nazi..
And many other games made a german solo, like the recently released STEEL DIVISION where you can take control of 21 PANZER in normandy and push back the americans out of france.
there is absolutely no reason, except being lazy, to only do an allies campaign.