There is a balance of resources, shelter, and number of species inside the region which is under consideration. While, the moon jelly's population may increase due to lack of presence of the competition for the resources that are present by means of other consumers of zooplankton that would enhance the growth of the moon jelly's.
Along, with that the decrease in the number of predators which are mainly the big marine animals inside the region that would increase the level or growth of the moon jelly's.