The prohibition on advertising applies to most if not all professionals who have professional bodies to represent them rather then a trade union e.g doctors, solictors, barristers,etc.
I think the point above about gentlemen and commercialism was one of the original reasons, as in the past professions would only have been accessible to those in the higher classes, and it would have been poor ettiquette to advertise one’s services.
I don’t know whether it should still be the case, although having seen some of the advertisements of lawyers and doctors in the states, i wouldn’t welcome them here.
Also, the lack of advertising means that there is less competition amongst firms for work, which allows architects to use the scale of feees agreed by their professional body, the RIAI here in ireland.