I'm going to say something that is reasonably unpopular ...
First Person Shooters are (in terms of gameplay mechanics) one of the easiest games to create in modern times. Generally speaking you're going to use a pre-built engine with all the tools you need to build the game; on top of that you can get a (pretty good) idea of what your audience wants by looking at the types of Mods people are developing.
Where the difficulty in producing a FPS comes in is they are amazingly complicated projects to manage due to the number of features people expect to be included. Unlike other games (racing games, fighters, etc.) your multiplayer game is not a minor addition to the core product, you have to develop seperate multiplayer maps for each mode of multiplayer you provide; with few exceptions, multiplayer is not an option.
A platformer (on the other hand) is much more difficult to develop because you're expected to come up with unique and interesting gameplay mechanics without much user generated content to be "inspired by". The projects are usually much easier to manage due to the very focused nature of the games.
I don't think it is beyond Nintendo to produce a high quality FPS or Bungie to produce a decent platformer, but I would expect Nintendo would have an easier time producing a FPS that sold 6 Million than Bungie producing a platformer that sold 5 Million.
