Controlling the way people look is one way to control them, and it's all about control. It's as simple as that.
They used to say it was because it wasn't acceptable "in the corporate world" which is what they wanted Witness men to appear to be, I suppose so they would look respectable and people would take them seriously, but now, men in the corporate world wear neatly trimmed facial hair, beards, goatees, so that's not a reasonable explanation.
I've heard that some of the older GB members associated facial hair in the 70s with the gay pride and hippy movements, and that was the real reason that they banned it. But, Knorr also thought blue jeans and sneakers were part of gay or hippy culture, so that's why they still have a ban on blue jeans at Bethel.
It's all about those old dudes in the suits and whatever blows their hair back on some levels. We're supposed to believe everything they say is right from God's hind end, he farts, they listen and then they fart on us, or something like that. LOL