Part of this may be related to the lack of direct monetization opportunities from Flickr. If you take the example of Instagram, the enormous number of users they have have created, "Instagram celebrities" out of thin air, with people securing modeling contracts or product endorsement deals simply by being popular on the network. I'm sure some photographers are getting business opportunities by posting their work as well, although that has a lot to do with how good they are at getting themselves exposure.
What the world really needs is another platform for vapid promotion :-)
Flickr/Yahoo! have taken some steps related to allowing users to monetize their photos over the years. There's been an agreement with Getty and they've done some other things as well. At its heart, though, flickr started out with very much a community orientation and, while they haven't necessarily done the best job of it, they've maintained more of a community than a commercial flavor.
Aside from the young, female, and attractive though, it's pretty hard to commercialize a flickr presence in any significant way. Even microstock sites which are explicitly about selling aren't a great source of income for most.
Even if