I think that Chris over at Movie Marketing Madness correctly pinpoints the problem with the posters for The Simpsons Movie: they aren’t specifically about the movie; they are all just general posters featuring The Simpsons.

The big question the marketing push for this movie has to answer is “Why should I pay to see this on a big screen when I can see The Simpsons for free on television all the time?” And so far they have done a poor job of making the film look special and unique, at least with the posters.

(source)