OK, so here's (albeit probably far fetched) viewon why Nintendo didn't show any core games minus AC.
So as you all know, E3 is neutered. As someone on GameFAQs calmly pointed out, it's now aimed for journalists and stockholders.
Now obviously with the Wii winning this petty console war by a large margin due to it's family friendly philosophy. By showing Wii Music. This will be a hotcake to the casuals. They're happy, shareholders are happy, Nintendo's happy. And of course, they are still developing games for their hardcore as well. They probably thought E3 wouldn't be a good place to show them to stockholders.
Now obviously with the 360 in second place and the PS3 struggling further behind, MS and Sony NEEDED to show SOMETHING good to make a come back. For MS, it was Final Fantasy XIII. Getting a third party game of a very popular series was their trump card. Otherwise, MS's conference would too have also been mediocre.
Long story short: Nintendo was pleasing there stockholders by showing that the Wii's family friendly orientation will keep money in their stocks.
That's my two cents anyway