The following information comes from Masahiro Sakurai's Twitter account...
- announcements like Smash Bros. Wii U/3DS would usually be accompanied by a trailer, but the team wanted to keep gamers patient while waiting for E3
- this is why they put out just a limited announcement
- Sakurai says E3 will come with a much better announcement
- the plan is to show off both the Wii U and 3DS versions at E3
Also worth noting, Sakurai exchanged tweets with Pokemon director Junichi Masada. The pair discussed how they're looking forward to seeing/hearing more about each other's games (Smash Bros. and Pokemon X/Y) at E3.