Its the same reason why female comedians aren't as good, to be great requires feedback and because of sexist attitudes they are given a half experience by men who are mostly just humoring them and not actually trying to teach them something. You also have to take into account that record companies basically force female singers and rappers into being sexual to sell records which makes the quality of their music one directional and thus not as good. Lastly since most women like pop it has influences on their music and everyone knows for every good pop song their is a 1000 that are complete garbage. All these add up to bad music.