Games which nail gameplay to such an extent and find a wide market are so rare that usually if a person or developer releases one they will probably never release one again. Its the hardest thing to do and yet they keep pulling it off. Its easier to make a game like Mass Effect because the rules and expectations are written for you than it is to continually make Mario the number one selling identity on the home consoles.
Nintendo are simply the best developers for nailing the gameplay and their 'flops' sell over a couple of million. You can ask why Naughty Dog games or Lionhead games etc don't sell as well and the answer is that they are simply nowhere near as talented at creating good gameplay.