I guess it’s because their part in the story is done.
Cutting off hair in ancient Asia (Japan, china, Korea & possibly some other Asian cultures) symbolizes being banished or rejected from their home. In the more modern age that is now, cutting long hair into a short cut means to forget the past, leaving the old and starting anew.
I seriously want to cut my hair, this long hair is wearing me off. :| :(( FML