Because the thing that you were looking for was a personalized feature (that you wanted for yourself) which grouped together posts/communities of /r/metal, /r/numetal, /r/djent, /r/metalcore and /r/deathcore.

Now the thing is the metal community is not a homogeneous community (brotherhood?) which likes the same bands and has the same tastes. Different people have different niche genres and it's only when people explore different genres do the people really understand what genre a/other band is. The more metal you listen to, the more able/experienced you are to divide bands into sub genres/ categories. I remember your posts/comments from a few months ago; you really don't know how to categorize metal, turning to Wikipedia for genre definitions (which is okay sometimes). You have to generate a listening ear yourself to categorize metal. People might call you an elitist for segregating and dissecting each band and their music, and that's okay; fuck them. Categorizing music makes it simpler for explaining the band's sound and their influences to another metal listener.

Maybe it's just me and my listening habits, but never have I ever gone from listening to a Black metal song, to Nu - metal, to a thrash metal song, to Deathcore, to Sludge, to Metalcore, and then Doom. I can't imagine the mindfuck /r/metal would cause me, if the front page incorporated these genres. Metal itself is a huge genre with various sub genres, and I'm glad that all of them aren't always posted on /r/metal and stay withing their respected sub reddit which one can visit if they feel like exploring. Only the best and finest of the respected sub genres [Doom, Death, Sludge, Groove, Black, Thrash, Folk, Melodic] are posted/upvoted on /r/metal. Well, most of the times.

Also the term "discouraged" for posting Metalcore, Deathcore, Djent and Nu - metal is more of a polite of saying "Not allowed". A lot of people respect earlier metalcore/mathcore (Converge, Dillinger Escape Plan, Coalesce, The Chariot) and deathcore (Whitechapel, Ovid Withering) and some Nu-metal since for a lot of them these were gateway bands into "True" metal. But what it has turned to recently is just an extremely polished and over produced mess, that caters to the masses. Devoid of any actual punk/hardcore rebellious themes. It will never be upvoted on /r/metal and almost certainly removed if it doesn't fit the "metal" mould.

If you feel like exploring various must listen albums of metal sub genres, this list is a list of recommendations from mods and esteemed users. and Essential Heavy Metal Albums. Go through the comments as well since people have posted albums that should have made it. Also, since you're new to metal (I think), check out /r/metal 's best of 2013 and 2014.