To the original question :
'Why don't people like Good Music' and 'why don't people like death metal' are two different questions. Everything that's been said about poop-music listeners on this thread, about the herd mentality and short attention spans, is true. To the poop/mainstream listeners it is not as much about the music, as about the pumping/dancing. That doesn't mean that all mainstream music is bad, it simply says something about the MTV audience.
But you forgot about people, who, say, have come into music through classical or jazz or prog-rock or something else, and are accustomed to a certain level of complexity and finesse in their music, which cannot be satisfied with most metal, which they find static, repetitive and dumb (I know such people personally). They simply cannot appreciate metal for what it is, because other musics give them everything and more.
I believe, one has to grow up with metal to really understand it, one has to ask the same questions at some point in one's life, that are the key questions of the spirit of good metal. Metal has a special kind of energy, which is driving (that involves the body into the music), and has a deepness (which involves the brain). So yes, entertainment is part of metal, and music is part of metal, so anybody who is only into one of these things, will not appreciate Good Metal.
D Mullholand, a dead composer
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NP: Jan Garbarek - Madar