Probably because Hubpages is a site that requires a higher level of maturity than other sites. First of all, you need to have something to share AND have the ability to communicate it effectively AND the desire to do so. This is strongly lacking in the younger generation who are more interested in sharing funny pictures and anime and stuff. How do I know? Because that's what my younger sisters do (Ages 23,17,14) all the time on Tumblr and Facebook.