I can think of either two things;
1. They have no interest in reactivating the Kindergatern on earth, at least, because they still think Rose Quartz is there. But, they still want to do stuff with earth, like use it for the cluster, science experiments, a communication hub, or just a warp hub.
2. They kinda figure that there's nothing left of the army and are waiting to exploit the earth as soon as the Crystal Gems are wiped out on earth. They have to have been on earth at least once in order to create the cluster and at that time, they probably knew there weren't a lot of crystal gems left from whatever method (like maybe a spy item) and they are just waiting for them to be destroyed.