Because like most features that Nintendo brings to the market, it sent it to die the day the console was released. Like the rest of the OS, it's slow and lacks basic features that should have been there from day one. Also like the rest of the Wii U Nintendo barely talks about it.
Miiverse is constantly being updated with new features, and more and more games are starting to use it in pretty cool ways.
Plus user-created communities should be coming soon, which is neat!