The hair of beard is full of tumeh'diga nitzotzos - so they wanna get rid of it.
Just to set the record straight. According to Kabbala of the AriZal. Any hair on the head (perhaps only near the top) is associated with יניקת החיצוניים.
The energy of the brain that makes it through the scull is similar to the leftover bones after a party that are served to the dogs.
Therefore, those who's traditions don't have an emphasis on growing פאות long (such as Chabad) cut thier פאות short to the halachic requirements, as well as cutting the rest of the hair on top of their head short. (We are familiar with communities who take removing the hair on top of the head very seriously for the very same reason, yet their leaders make a big emphasis on growing their פאות long, ותבא עליהם ברכה. )