It often doesn't really matter what a movie's actual quality is, as its success is judged based on how it did at the box office. This isn't always a terrible metric, since it means that people obviously were interested in the subject matter, but sometimes it also means that even mediocre movies can get a sequel instantly greenlit based on popularity alone. The studios know that if the first movie made a ton of money even with a half-baked script or premise, there's no reason they would need to put a ton of effort into a sequel in order to make just as much of a profit.