There's a number of reason about why TSM had poor legs. The first one was definetly that summer 2007 was an extremely crowded market. Does anybody remember it? Remember how for the two middle weeks in July, the media wouldn't shut up about Harry Potter, and I swear to god the minute that book came out they ran off and started printing story after story about the Simpsons Movie. But as soon as that came out, they moved immediately onto Bourne. Because of a lot of competition (re: Bourne, which while not grossing as strong as an opening weekend, did end up doing better domestically than TSM), including Rush Hour 3 and such, Simpsons suffered.
Two was that there was a large belief that everyone who wanted to see it, saw it on opening weekend. Roughly only 10-20 million people saw this movie on opening weekend, which is about the amount of viewers the series regularly scores. Anyone else either waited for DVD or didn't care at all. After opening weekend, not many others wanted to go. For example, if last Sunday's Simpsons episode reran this Sunday again, do you think many people missed it? Probably not. I think audiences treated this as a big summer episode of the series. They paid their religious viewer dues by going to see it the weekend of July 27th and that's it. Whoever doesn't watch the Simpsons didn't go, plain and simple.
Pretty much, it's a movie based off of a TV show that's still airing. Sure, the Simpsons are pretty big in America (I think the opening weekend proves that), but their popularity is waning and the show on the whole is still a bit cult, because it has a love it/hate it mentality. Thus, only repeat viewers and people who missed it because of vacation or whatever kept it alive afterwards. Plus, the aforementioned competition really did it in...
I still really think the movie benefited by 20th Century Fox's luck on picking the opening weekend. Every single other movie scheduled to play that weekend was pulled out, except for No Reservations (dumb move!)...and TSM really capitalized on that one. Not only was it the first big movie since Harry Potter, but it was the only movie that weekend, and what people (mostly the media) couldn't shut up about. On top of it all, it had good reviews! (suck on that, Transformers!) As a result, people flocked. But then Bourne came out next weekend. A sequel, with good reviews to boot! As a result, people flocked to that one instead, having already seen TSM. Harry Potter had at least two weeks to stay mildly afloat, as well as repeat viewers who were far more loyal than any Simpsons fan would ever be. Transformers was big, dumb action movie with special effects...that was released in the United States of America. Need I say anymore?...