It's also pretty impossible for Nintendo, or any videogame company, to really be reactionary in their development cycle. So, uh, the Wii U ain't selling as well as they would like. Well what can they really do about that? The games that are due for 2013 were already planned well ahead of time. They can't just hammer out a decent game in a few months in response to the market. What we'll see at E3, and for probably all of 2013, was already fairly planned out before the Wii U even launched.
I suppose Nintendo could show trailers for games that are in early development that aren't due for a while to build some hype of the future, but they haven't done that sort of thing in a while. They typically don't announce anything that E3 that is not intended to be in stores prior to the next E3.