There is no reason, and there never was.From my point of view I can see no reason not to fit wall switches, providing they are outside of zones 0, 1 and 2 (and are RCD protected etc. etc.)
Previous regulations only changed the distance switches had to be from the bath, switches were never prohibited in bathrooms.
In the 1960s, it was stated that switches had to be 'out of reach' of someone using the bath, later it was changed to specified distances and various defined zones.
For those that believe the switches can't be used due to the damp environment - the exact same switches are used in kitchens, utility rooms, cloakrooms, sheds, carports and garages.