Because metal is pretty lame.
Now, there's nothing wrong with being lame, I've spent several decades now doing that and it's working out pretty well, but you've got to own your own lameness instead of pretending everyone else is just too lame to realize how cool the lame stuff you like is.
Okay, now I've alienated half the forum, why have I done that?
Essentially, metal as a genre (and I know there are exceptions to every rule, especially in a genre which is genuinely international and as diverse as metal) consists of a bunch of people, overwhelmingly white (at least in Europe and the USA) and disproportionately middle class, who, for some reason, deliberately set out to distinguish themselves from mainstream culture, who affected a style of dress, music and even aesthetics which emphasised rebellion against tradition and conformity.
Rap as a genre (and in this case, there are actually a lot of exceptions to the rule) consists of a bunch of people of different ethnicities and backgrounds, but in my experience what unites a lot of them is that many never had the luxury of being able to distance themselves from mainstream culture. They are people for whom fitting in is a matter of emotional or physical survival. It speaks, not just to the black urban audience it was originally designed for, but to anyone who feels betrayed or disadvantaged by the modern world, a world which constantly tells you that material possessions and hedonistic excess is the path to happiness, but who doesn't have the security to simply detach from that world into a nice, safe fantasy. In that sense, rap is more brutal than metal will ever be. Even when some dickhead is just talking about how much money he made selling crack and how he's going to buy loads of nice cars and have sex with your girlfriend (specifically your girlfriend, for some reason) he's still ultimately talking about a world in which that constitutes the only means of winning, and that world is harsh because it is in some ways the one we live in.
I mean, by "we" I mean not me specifically, I'm white and have a trust fund so of course I listened to metal, and you bet I thought I was better and deeper and more thoughtful than everyone else because I did. But then, I had the luxury of being allowed to think like that. I chose to be an outsider to mainstream culture, and I did that because it's actually a lot easier than trying to live within it and failing.