Because he has to deal with The Q Ball factor.
I prefer the answer that it's because the actor that was chosen to portray the character (Patrick Stewart) is bald, therefore the character is.
The official answer is that, by the 24th century, nobody cares. You may laugh, but that is apparently Patrick Stewart's offical stance on this issue when asked at conventions.
This was also Gene Roddenberry's answer on the subject
Agreed, it makes sense. By the 24th century, no one will care. Hair care would have progressed to the point where he could have a full head of hair, but to him, and the women he consorts with, it is irrelevant. Bless the future and its utopian goodness.