Well, nems are very difficult animals to keep. They require pristine and stable water conditions, where are very rarely found in tanks younger than a year old. They also have very high lighting requirement. And you run the danger of nuking your tank if it dies in there.
They can also sting and kill your corals if they grow too close or if the nem decides to take a walk
Plus there is no guarantee that clowns will even be hosted by the nem, your clowns might decide that they like another location better.