Because, the way his beard and moustache are trimmed s the Islamic way.
Here, from an Ask the Imam website:
Question: Is there a limit to growing facial hair, since some people have very thick hair which sometimes goes from the chest to the neck and even all the way to the eyes?
Albani's answer: It is not permissible for the Muslim to remove any hair from the beard except what the Lawgiver has allowed. It is reported from one of the companions who narrated the hadeeth, "Leave the beard and trim the mustaches," 'Abdullaah bin 'Umar, that he used to remove from his beard what was below a fistful. Apart from that, trimming the beard is against the Sunnah, whether or not the man's beard is pleasing to him, and whether or not it is pleasing to others