That's because Nazi sympathizers and white supremacists are naturally drawn towards WW2 fantasies where Germany is portrayed sympathetically or treated like they had a chance of winning.
Yawn. I find it so tiresome how big of a stick our society has up its ass about Germany in WW2. This obsession with being anti-Nazi seriously borders on cult-like. It's been nearly 80 years. And you cannot expect me to take seriously the claim that 80% of the WW2 video game fandom are all white supremacists (and who cares if people you don't like like your video game, anyway?). People like Germany because they're cool and had the most badass tanks and uniforms.
But this is kind of a rabbit hole, anyway. Other games have had German campaigns, including in our very own beloved series. I see absolutely no reason why we can't have one in CoH3.