According to this article here, director Neill Blomkamp would be interested in making a prequel to District-9 but not necessarily a sequel. His reasons, he said, were the style in which the film would be shot. In his vision of a District-9 sequel it would be a traditional type project – alien shoot ’em type thing – while a prequel would open up avenues of complete artistic freedom.

Whatever his reason, my two-cents would be that the story of the aliens coming back to not only liberate the captives remaining on earth but also to fulfill the promise made to Wikus is a story line with great potential and not to make a sequel and finish the story would do all District-9 fans a dis-service.