That is definitely not true in seattle. My best guess is because a lot of the power/prog tours skip here. Death metal shows usually draw 100 kids, maybe more if it's a crossover appeal with the hardcore/emo crowd, but rarely sells out.
The most crowded shows i've been to are the following:
Kreator
Destruction
Overkill
Sonata Arctica
Kamelot
Blind Guardian
Iced Earth/Jag Panzer
Stratovarius
HammerFall/Edguy
The first three are thrash and lean more towards extreme metal, but i'm just saying that you go with a band like Suffocation who gets magazine covers all over so their popularity is indisputable, they can never pull a crowd half the size of power metal bands.
And trust me, seattle doesn't feel like a straight up heavy/power city at all. I have no idea why this happens