Honestly, this is about as simple a question as you can get. Why does Nintendo continue to sell games at full price? Because they can.
If you own a Nintendo system, what are you going to do? What are your alternatives? You're going to take what Nintendo gives you and you're going to like it. It's not like there is a vast array of choices on Nintendo systems. This is especially true if you're a big fan of the cutesy style reflected in many Nintendo games, as not a lot of other studios work in that direction. You'll simply have to drop trow and accept the pounding.
This is kind of an extension of the business philosophy they had during the NES era, when they would purposefully reduce shipment size in order to spread sales out over the year and to keep their own games selling at full price while keeping third-party games down. When you manufacture all the games that appear on your system, you can manipulate the situation as you like. It was brutal but also ingenious.
