Why is it that people put down rap/hip-hop music?

Okay, being a long time fan of rap music, especially dating back to my childhood (Believe me, during that time, I wasn't aware of explicit lyrics all the time because I heard radio edits, especially those radio edits that were re-recorded with different lyrics), I wonder why people always have to put down the genre of music. Personal preferences and appeal isn't the matter here. If one doesn't like it, fine. But if somebody has an ignorant reason, example stating that it only describes violence, , drugs, etc., not to mention the ignorant statement saying that isn't exactly music. If you don't like it, fine. But what I wonder is why it never gets respect. It's been around since the 1970s (Believe it or not) and it still gets no respect. If you think it's all gangsta and stuff, try listening to stuff before N.W.A. hit the scene, like Public Enemy, Run-DMC, Boogie Down Productions, Afrika Bambaataa, Kurtis Blow, etc. and tell me that it sounds "thuggish". As for more contemporary stuff, try listening to guys like Mos Def, Common, Talib Kweli, The Roots, Lupe Fiasco, Juric 5, Immortal Technique, etc. Again, I don't care if doesn't meet one's taste buds, I don't care if someone doesn't like it, fine. Whatever floats your boat. But what I don't understand is the narrow-minded and ignorant reasons to put it down. Honestly, I may listen to old school stuff from N.W.A., Snoop (Death Row and No Limit days), Too Short, etc. but at least it sounds much better than the stuff on the radio right now. The subject matter may be what I mentioned before, but at least it sounded much better than the played out stuff of now. Rap, to me, is an acquired taste. I just want to know what gives a reason. Apart from any ignorant umptions, just state a reason. I don't care if someone doesn't like it. Besides, I like other genres, too. Rap just happens to be my main genre that I prefer to listen to.