Rap seems to change with every generation or goes through changes. Usually the people that came up in a specific generation of it tend to stick to that as what they see is the best. So the newer version gets shit on. When the new/young people get shit on they fire back with "you old, you dont understand"
Also those other genres tend to need a mf to know how to play an instrument or something and that weeds alotta mfs that would prob contribute to them having this same discussion. For rap, you dont even necessarily have to be good to blow up. Buy some good beats, have a catchy hook...you half way there.
People that came up in the earlier days dont wanna hear that, because there was a different criteria to even get through. You had to have something usually. Now you can do your own shit and throw it on YouTube or sound cloud, and let it spread.