I think the reason that rockers don't like rap and 50 Cent (among others) is because of a talent issue and the redundancy of rap.
Almost all rappers don't play an instrument, which right there sets a musician apart.
Almost all the songs are about the same things ("gettin' down" - 'dancing', "goin' down" - erm. . .felatio, "hoes", or how much money/cars/guns/G's/bling/other material items they've acquired), which tends to run dry after a while. It doesn't make the m(a/e)n a genius, it makes him popular. Which is also a misconception. An entire society of people who praise one person doesn't make the person smart it makes him popular, or in rare cases, everyone else dumber.
Plus you have the whole "I featured 20 artists on a single album on every album and I'm on 20 other G's albums" factor which comes into play. That gets your name out and gets people to know who you are.
Then you just have the background music of rap. The background music of rap is what I know most people listen to; it's the tune that gets stuck in their head and rarely the lyrics. Every once in a while, a breakthrough rap "artist" will hit it big in the scene and will submit something novel or worthy of being noted, like Eminem, who often rapped about more than just hoes, bling, and the ghetto (often referring to conservative America, his mom, and most importantly his love for his daughter).
Rockers and fans of rock, however, are often portrayed as the ignorant-minded metal-head who just likes loud noise. Well, let's clear this out of the way: They don't. Many people who listen to it, and all those who compose it, are also musicians. This means that they not only write their own lyrics (not all, but some), but also [usually] play guitar, bass, drums and whatnot. With rappers, though, they do not play an instrument. They talk into a microphone, flaunt how much money they have, and talk about how pimped out their American luxury cars are.
I guess it all comes down to arrogance. Eminem made over $300,000,000.00 between 1999 and 2001. Now, he's a very respected man amongst other musicians and is quite prominent in the music industry. Now why is this?
Well, Eminem has gained respect for a number of factors, even from the likes Marilyn Manson (shock rocker) to Elton John (flambouyant homosexual soft-rocker) , and even dated Mariah Carey (banshee-wailing poptart). He doesn't exude much arrogance, either, and I think that's what it comes down. Because he's collaborated with very different people in live shows, he demonstrates that he accepts people for what they are, whereas Ludacris, Jay-Z, Snoop Dogg, Sean "Puffy" Combs, and 50 Cent all show off what they have and rub it in peoples' noses.
Since rock artists don't do this (99.99% of the time), they're fans generally regard them as much more humble, accepting, and even understanding, in contrast to the image that rappers havebeing "suck it or die" or "I have (way) more money than you".
I hope this has helped you understand why many people who listen to rock often intensely dislike rap artists. And I may note that this is also comming from a(n) [ex] rap fan.
p.s. : Sorry about the length.
EDIT 1
Eminem.. He named himself after a choclate sweet.. great.
Actually it's his initials M.M., for
Marshal Mathers.