Why male solo is not as succesful as their female counterpart in Japan?

[Surreal17]

This is a topic that has kept me interested for a while. I mean, I've never heard of a Japanese male singer as succesful as Ayu, Namie, Kuu, Utada, YUI, Shiina Ringo etc. Or someone with a huge relevance or always changing image.
I know that Ken Hirai had some nice sales at some point but his music is a no for me. Anyone else?
Its really weird because Korea has successful male solos alongside other categories but Japan is mainly interested in female solos (I know, not so much now), girl & boy groups and rock bands.