I'm no longer a JW, but it was the same here in England. It was explained along the lines and principal of women having to wear skirts or dresses and not trousers. (I dont think I owned a pair of trousers till I left!!)
It is how the "world" perceives the JW, beards are associated with the world as being "way out", with "hippies" etc and it doesn't give a good impression. Bit like wearing a suit to go knocking on doors and at KH, it appers smart and apparently beards don't give that impression.
Well that's what I remember anyway.
Any other thoughts appreciated