There is a decent number of people who tend to play a generation behind when it comes to video games. For people who might not be willing to shell out $300+ for a Wii U, a $99 price point can be a lucrative offer for a Wii Mini that has a vast library of games that are still easily found at stores, that still hold up well and are relatively modern, and that you can get often buy used for a fraction of the price they were originally sold for.
Growing up, I don't think my family ever owned a new console on release. I know we had the re-released/updated NES and SNES pictured above. We didn't own an N64 or Gamecube until well into their life cycles, and I didn't get my hands on a Wii until Skyward Sword came out, five years after the Wii released.