People just hate rap, because it's popular, and people think hating whatever happens to be popular makes them look hip or intellectual.
Now you're just postulating.
The thing about rap is, more often than not, that it is what becomes popular. People tend to dig the simple music and rhythms and the rappers attitude, expressed through more or less clever rhymes. (At least that's my general assumption.)
The reason I dislike rap is mainly because I find it very flat and uninteresting musically. It seems to me that the main focus in rap is the lyrics, not the music. Rap lyrics are longer than anything else - often more personal than anything else - which requires a lot of work from the artist. Admitted, some tracks here and there got pretty catchy music behind the lyrics, but at the end of the day, it's still the lyrics that's dominating.
I listen to music mainly for the music itself and not as much for the lyrics. I guess that's probably the reason why I've settled upon metal and rock. Lately, the more progressive, complicated and (to some degree) obscure it is, the more I like it. Why? Because it shows there's been put a decent amount of work into the riffs and it requires an incredible talent to be able to write pieces - hell, even to play like that. More often than not, the music itself also creates different moods and feelings you won't find as much in rap, in my opinion.
I won't say rap is the future. In fact, I think it's a bit naïve to think that. I get the point that the older generations doesn't like it - and because of that you could say that rap is a part of the younger generations (which leads to think it's futuristic) but that is seen in a lot of different genres. Rock is a good example here.
The thing about music, and art in general, is that it constantly moves and evolves. People who like more "core" music genres might not like the way the genre has developed and therefore dislike the new and popular music that follows.
And new forms of art appears. Like rap. And dubstep. And even progressive-extreme-avant-garde metal!