It's not strongly plot-driven, which is what tends to give longer movies their staying power, and are also what help you know implicitly where you are in a movie. With "In the Heights", it's tough to identify what the core journey is, and thus where we are in that story: is Abuela dying the climax? Is the impromptu Carnival the climax? Is the dancing on the side of the building the climax? No, wait, now we're seeing Usnavi finding the lottery ticket, is this the climax? Whether we can identify this phenomenon or not, that's a big part of why we can feel that restlesness of how long the movie has been and how much further it will go.
Credit to Hudes and Miranda, they've opted for a quite loose structure that gives us some semblance of a plot up until the blackout, but that really goes away and we're left with this marvelous tapestry of life. To me, it feels like a Bergman film with better songs, which is exciting, but I think a challenge for mainstream audiences. And if we're asking ourselves why the film hasn't opened well, I think the fact that the trailer couldn't identify and communicate the story being told is a huge part of that.