Nintendo consoles are popular with kids, and they get them all the time for their birthdays or Christmas. I have no doubts saying that children make up a large amount of the consumer percentage for Nintendo. Think, if you were a parent and you knew nothing about gaming, and you were on a low budget, would you buy the 3DS version, which is cheaper, or the Wii U version, which together with the console would be 360$ total.
Or another scenario, the 3DS version has come out and you've bought your child the game. He enjoys it, but now there is a Wii U version coming out. Once again, you are a parent who knows nothing about gaming, are you going to buy your child the same game on a different console?