That's one possibility; another is that their license has simply expired. Remember, MegaBloks also had the license for Cars, but LEGO has it now, and so far there's just a single feature film in that franchise (plus shorts, but those aren't specifically the basis for most of the sets from either company anyway).
Actually, the first wave of Indiana Jones sets covered scenes from the original movies, while the second wave was devoted to the new movie; that's pretty much what happened with Toy Story as well (the second wave does have one set based on a scene from Toy Story 2, but otherwise it's all about 3). Based on those two examples, I presume the first wave of Pirates of the Caribbean sets will cover the first three movies (or the series in general), followed by a second wave dealing more or less entirely with the new movie. If that's the case and if the theme lasts beyond those first two waves, I imagine the subsequent waves will deal with all four movies to varying degrees, as happened with the third and fourth waves of Indiana Jones.