I'm guessing it's just taking a conservative approach. We worked on one device that excluded pregnant women from indicated use. There was no physiological rationale but they didn't want to chance it.
So I think it's probably more the opposite: if you do indicate it for use with a certain population, you'd want the clinical evidence to show safety and efficacy. There's a chance that after clinical trials or use that you do find out that use isn't indicated for a population and you need to narrow the indications down but otherwise, it's just eliminating risk.