Why do rock fans seem to be alot less tolerant of rap, than rap fans are to rock?

for example, if you youtube a rap song, a majority of the comments are aimed toward lyrics, and comparisons with other rappers.there is also alooot hatred for mainstream pop, such as justin beiber and lady gaga.however if you youtube a rock song, they constantly seem to degrade rap, even going as far as to say it isnt music.not only is that, but they also share a similar hatred towards mainstream pop.i find this especially curious, seeing as how some of the greatest rappers incorporate rock into their music, such as run dmc, cypress hill, and dr dre.yet if a rock band incorporates rap, they are often shunned, such as linkin park, rage against the machine etc.this is just my opinion, please respect it, even if you dont agree.