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Most metal sub-genres display such a level of similarity in rhythmic, timbral, melodic, instrumentational, harmonic, and lyrical content that, in almost any other style of music, it would all be lumped together.
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I love the quirky, silly, barely distinct plethora of Metal sub-genres, some with only a handful of bands in them, but it IS kind of silly how we sub-divide.
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For Example, look at contemporary "classical" music. That's some of the most varied stuff out there. 70 years ago, you had everything from guys trying to sound like Bach with just a few changes; you had guys applying mathematical formulas to their music to ensure that no note, rhythm, or harmony was repeated; you had guys composing music with nothing but the sounds of passing trains; you had guys letting the randomizing principles of various fortune-telling devices write for them; and you had guys composing nothing but silence. You had guys starting to not even use the notes you're all used to.
From there, "classical" music has only expanded and gotten weirder and more varied, but it's still usually only broken into a very few sub-genres.
Same with contemporary Jazz...
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However, Metal is almost always in one of a few simple time-signatures, almost always has very similar instrumentation, uses the same very simple harmonies, the same musical concepts, similar melodies, the same playing styles, the same dynamic, etc, but still somehow has tons of very similar subgenres...
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I think a lot of it comes from Metal-fans being very exclusive, and often (in my experience) really not listening to much other than Metal, and maybe one or two other genres that have some reasonably related aspects.
That isolation from other musics, I think, leads to a desire to create variety within the genre, without really expanding it's musical borders too much.
Please don't beat me up.
That said, Death and Thrash are pretty dissimilar, as mainstream metal genres go. Listen a bit more, I'm sure you'll hear the difference!
