Yeah, that and the fact they were filming the movie at the time... if they rushed out a teaser, they probably could have got one in before Tron, but that would have been a long shot. And to unveil a big movie like that, you're going to want to do it on a movie people will actually see.Given the goldfish memory of the public, I think you're right. This isn't the same as election year cycle media blitzing.
I went to see Hoodwinked 2 as solidarity for the writer of the true Fraggle Rock movie that's never going to happen, and you could tell someone knew the film was going to flop. The only trailers attached to it were of lame Walden Media type kid and tween movies based on books I never heard of that look like they had extremely low budgets... and the Pooh trailer that was out for a while. The trailers that were new were unimportant films that are clearly getting a theatrical release as a contractual obligation, but will make their money back when they hit DVD.
Putting the Muppet trailer before Jungle Cats, Prom, or... that other thing (makes the face characters on Soul Eater make when they see Excalibur upon it's mere thought) wouldn't have been a big blitz or even much hype for the film. Pirates? Disney's first summer blockbuster contender in May? Perfect timing.