I like rap under the right circumstances. It really works in film like chase scenes, fights, etc. -very exciting.
I think a reason it prevails is that no one has created a replacement music form. Also, rap songs usually have a well defined message i.e., it is about something the listener cares about (whatever that may be).
Finally, watch the movie "Myra Breckenridge" from 1970 and see Mae West (related to kanye?) doing the first known rap performance!
This makes rap 46 years old -not 30... (hey kids! Why are you listening to "old fashion" music? C'mon, invent something of your own!).