Because metal is supposed to be as "underground" and unmarketable as possible (while being as complex as classical music), otherwise it's pop shit and objectively bad music, according to the folks at Death Metal Underground and similar people that think extreme metal is the only true metal. And the only political message that metal is allowed to have are Nazi, satanic, pagan, radical right or anti-communist lyrics. I mean, just look at the Nazi black metal scene.
Nevermind the fact that there have been a ton of protest songs throughout the whole history of the genre, criticizing war, propaganda, and the American society, government and patriotism (even the entire style of thrash metal was born from punk influences and protest music against injustice).
Metalheads, especially the extreme metal kind, are the ultimate contrarian hipsters, likely a consequence of being social outcasts in their adolescence and in the early decades of the genre, which fascists have been appropriating since then. Also, metal fans in general are extremely pretentious when it comes to the musical aspect.