Why are harts (male deer) not allowed to be hunted in Sir Gawain and the Green Knight? Is there a significant reason for it? Or a function perhaps?
The lord of the castle instituted a closed season on harts, or male deer.
"They let the harts with high branched heads have way,
the brave bucks also with their broad antlers;
for the noble lord had bidden that in close season
no man there should meddle with those male deer."
Closed seasons are usually to preserve the herds so that the lord of the castle never runs out of prey to hunt. On another note, you might be interested to know about St. Hubert, the patron saint of hunters during the Middle Ages. Legend has it that he converted to Catholicism after seeing a male hart in the forest with a crucifix between his antlers while hunting on Good Friday.