The main reason I understood that it was not recommended is that it does not represent the look of a minister of God. They said that if our appearance were a stumbling block to others it was unacceptable. In the 60's in the U.S. a beard was a symbol of rebellion. As ministers we would not want to portray rebelliousness like Satan.
A brother I knew in the 1990's who came from Germany originally had a van dyke beard. They were on him about it and he said, "its OK in Germany and there are no hippies anymore so I'm keeping it". He did, but they wouldn't let him serve in any capacity. He said he didn't really care about the beard but he loved making the brothers crazy so he kept it. My kind of guy. He was a born in too by the way. He had a stubborn German streak in him.