Well, it's not like they can't, it's just that it would be too drastic of a change for them if they don't have the expertise in it. The problem, I would think, is the market itself. Why would Nintendo lump itself into FPS when it is the most competitive genre right now? There is far less competition in action adventure, platformers, party games, RPGs, racing, and casual games. As far as I can tell, Mario has no competition when it comes to platformers, Zelda has some competition with action-adventure(apparently its an ARPG to some people), but if you have no competition in an area, just stick to that and milk it while you can.
Nintendo going into FPS genre would really be them catching up or either innovating, which will be much different than what a 3rd Party would put out. The market is just too competitive, I think, for them to join it, but not saying they wouldn't however. Metroid is still solid and seperates itself heavily from other FPS, but new IPs from Nintendo, I guess it's a wait and see what their other studios put out. We got Xenoblade Chronicles, which is a good core game, so perhaps stronger RPGs will hit the Wii U.