Musically speaking, I'd say it's because current hip-hop and hip-hop from the 80's, possibly even the 90's are so vastly different that they're barely the same genre.
Then there's the fact that there's some things people do nowadays that was practically against hip-hop's unofficial code of ethics. You see how fast people hop on trends and sounds that someone else made hot these days. "that producer got a hit? I need a song just like it from him!" "autotune singing on the hook is hot? gotta get that plug-in for myself!". I don't think oldheads that were around in the days when someone would get beat up for "jackin" raps or beats or whatever else will ever respect that.