I think in part because metal was king in the mainstream until
"Nirvana came along" as they say. Realistically, though, grunge (the
truly dirty, heavy stuff) was just a different variation or
distillation of the Black Sabbath sound. Nirvana were heavily
influenced by Sabbath, as were Soundgarden, Mother Love Bone, and many
other grunge bands (lesser known acts like Fudgetunnel and Tad owe a
HUGE debt to Sabbath). The difference was, unlike many of the metal
bands who were distilling Sabbath influences alongside Bad Company or
Led Zeppelin to create a melding of heavy rock and early metal into
something new, these grunge & alternative bands were filtering in pop,
early rock and roll, psychadelia, some funk, and varying other
influences (country, in some cases like The Meat Puppets), rather than
just sticking to the "old guard" of hard rock, glam, and early metal.
What the bands weaned on 70's glam rock, NWOBHM and early metal & hard
rock didn't realize was that there were other types of music they
could be influenced by. Bad Brains was just as influenced by their
peers in the hardcore punk scene as they were by the 70s funk era.
Why weren't they accepted? Simple - prejudice. Just like any other
prejudice, it stems from an ignorance of the style, sound, culture, or
some combination of those elements. There have been lots of musical
styles I haven't like over the years because of a lack of
understanding. Once I understood the basic tenets, it became much
easier for me to appreciate the music for what it was & what it was
trying to accomplish.
Really, this is no different than the current anathema of "core" among
old-school metalheads. Older guys would rather hear blazing Yngwie
Malmsteen solos and melodic riffs than some guy barking into a
microphone or yelling "Go!" every time the band goes into a chugga-
chugga breakdown. They don't understand why these "kids" can't "grow
up and learn to play their instruments". The thing is, they have.
Maybe not at the same level of proficiency as, say, Iron Maiden, but
they at least have skill enough to play what they are. That's what
they like to play, and that's just fine. I consider myself blessed to
have grown up & "discovered" hard rock music during the "changing of
the guard" (so quoth Dee Snider), when I was hearing bands like Guns
'n Roses, Metallica, Skid Row, and Cinderella on the air at the same
time I was hearing Nirvana, Pearl Jam, Soundgarden, and so forth.
These bands were all on the same rock/pop station I was listening to,
so to me, there was no real divergence. Did they approach the music
differently? Yes, certainly they did. However, all those bands
retained a driving guitar sound of one type or another, w/ varying
degrees of melody, harmony, proficiency, and songwriting skill. To
me, it was just about listening to a band for what they were, and
meeting them where they were at.
I think, too, a lot of metalheads are uncomfortable admitting they
like anything non-metal that isn't considered "cool" by metalheads.
Most metal fans wouldn't be ashamed to tell you like Alice Cooper's
early stuff, or that they're into Frank Zappa. Heck, many metal fans
would even tell you they're into The Beatles. However, most would
deny liking disco, punk, dance, rap, or anything else that would be
considered "unmetal" to like. I don't care - I'm a major metal fan
who also likes to listen to other genres. That's the beauty of music;
it's such a vast array of expression. I love listening to my iPod on
shuffle mode & hearing Mortification, then ABBA, then Bon Jovi, then
Jetenderpaul, then something from the Star Wars soundtrack, then maybe
some Chemical Brothers, then some Stryper. It's awesome when I can
experience the entire array of stuff in my collection in a short
period of time like that. If more metal fans were truly "open minded"
like that, the metal "scene" would be a far less annoying place to be
associated with. :)