Most likely reason being that your prescription will change as you heal, I would think.
I think its mostly more about not wanting to give you the wrong prescription before youre ready. I dont think they would do you any harm to wear…just that they dont want you to spend money on a prescription that will not be current once the healing is done.
Thats my guess anyway…If its not…as @Schizbro said, they probably have good reason. Just give them a little time.