In the United States having beards is not generally accepted as being well kept in the business world.
That's what the JWs will tell you. The problem is that it's crap. As others have mentioned, there are plenty of lawyers, doctors, accountants, businessmen, and politicians who wear beards.
I think the WTS has no business trying to control its members' grooming anyway. But even by the standards that it officially sets, the ban on beards is ridiculous.