That is one of the reasons. The centrally authorised route is only one way a medicines may be approved in the EU and is not the usual way for OTC medicines.I don’t think that’s the reason, EU pharmaceutical law applies to Northern Ireland since Brexit and prior to that applied to all Uk.
This link explains:
w w w.ema.europa.eu/en/about-us/what-we-do/authorisation-medicines#:~:text=All%20medicines%20must%20be%20authorised,route%20and%20a%20national%20route.
I'd post the link properly but I'm not allowed.