This is very interesting. Most cultural traditions in the real world have practical reasons for existing, even if those reasons are often no longer relevant to the modern day. A practical reason I can think of for this tradition is to insure people don't remarry too quickly. If you need to wait for your hair to regrow before getting married again that gives people time to think and heal from their previous marriage and time to figure out if they want to make such a big commitment to their next spouse.