Eh? They arent?
Maybe because as a genre; they have the following traits.
1. Should be
loud to enjoy. Loudness isnt loved by all.
2. Most high-tempoed, not easily understandable.
3. Transtitions within the song maybe difficult to digest.
4. Patroned by a bunch of cocaine-snuffing-emos.

5. Can't be played at wedding parties.

6. Not all people play/understand instruments. And so not all will actually give due respect to the drum, guitar, piano, whatevers played in the songs.
Pop, in turn does not need such an understanding.
7. Lyrics are much inhuman, un-productive and IMo worthless. though I do enjoy the songs.
metallica, iron maiden, thats enough for meh.