In the '70s there was a WT which mentioned beards in the same sentence as hippies.
In the 2010's there was a WT which correlated beards with pirates.
No-one wants to be a hippy pirate, so beards are banned.
Actually there has never been a letter, publication or rule to say you can't have a beard.
The nearest they come to formalising it is if you are a convention speaker then you can't have one.
Read the Theocratic School book and it just talks about being neat and clean.
Every ruling at a congregation level comes from an implied reading of disapproval from the WT's that I mentioned above.