Erm... because they are new?
TBH, the conditions of my zoas are average at best. There are many reefers out there who have awesome zoas that are thriving, which means, growing and multiplying very well. Good water perimeters, flow and lighting are major contributors to having healthy zoas, and I'm still trying to get there.
If your polyps tend to extend their "stalk", lighting might be an issue.