i know many hip hop groups and electronica artists several times better than a lot of rock bands. so please dont say rap sucks.
and i disagree about punk not evolving. the emergence of hardcore punk in the late70's/early 80's lead to a load of new music genres based on punk, like powerviolence, crust, grindcore, digital hardcore, and a whole lot more. unfortunately, most of those bands have died out nowadays, except for a promising few (im looking at you iskra)
I used to be an extreme metalhead, listening to the extremely underground black metal acts.
until a few years ago when i stopped being pretentious (in terms of listening to metal at least) and realized that raw black metal is horrible, zero artistic value.
now im into mainly hardcore punk and grindcore. i find the raw energy of the music stimulating and cathartic, more so than metal. i find grindcore more intense than death metal and after a brutal moshing to it, i find myself extremely refreshed.
that and many grindcore bands have surprisingly deep lyrics.
but i still really like metal.
and on the topic of the million sub genres of metal:
i know im probably bias, but metal IS very diverse, if you make a person who has never listened or heard of rock before listen to foo fighters(alt rock) and stone temple pilots(grunge), both different bands with different styles, they would most likely say that they sound at least similar enough to be put under the same broad category.
however, if you compare a band like iron maiden(trad metal) to a band like wormphlegm(funeral doom metal), a person who has never even heard of metal before would have no idea they both fall under the same category of music. just listen to each bands songs (although i recommend listening to wormphlegm in broad daylight on the toilet since it will scare the crap out of you.)
If i can enjoy music from both wormphlegm AND the roots, i bet you guys can too. stop hating genres other than rock/metal/whatever. music is just like people, belonging to a certain group/race/genre isn't what makes you bad, it's your own actions/songwriting that do.
but hey, i didnt stop acting pretentious since instead of being "YEAH! VLAD TEPES AND LES LEGIONS NOIRES IS THE BEST METAL SINCE THEY'RE THE LEAST KNOWN" im now "i am so high class that i can listen to ALL music, unlike you guys, *snort*". but who doesn't like feeling superior to other people?
oh, and as for the thread question, it's probably because people who listen to metal can really express their passion for metal outside because it's somewhat of a taboo genre due to all the offensive metal bands with no talent that mar the reputation of the metal bands that put originality and passion into their music. so they go to the anonymity of the web to express their appreciation for the genre, and they can't be put down as easily there. that and most metalheads are atheists and most computer geeks are smart and generally the more educated you are the less likely you'll be religious so they may find some sort of uniting factor there. though... i'd like to think im smart...