Many reasons.

Kids can wet the bed for many reasons that can be from scared of "monsters" to to tired to wake up to go potty. They could also have a some type of infection. Like Magen said if they are older then 6 it might be a good idea to go see the Dr. Ask the child why they are wetting the bed. You could have them help you take down their bed and wash the bedding and help re-make the bed. This might help if they are just being stubborn, but also might help it show them that they need to help out when they "make a mess" But have them help in the morning not in the middle of the night that just doesn't seem fair...

Moms Expertise
    I agree and I would add that some kids might just not be physically ready yet.
      Yes i have often heard the term immature bladder ... meaning they are just not ready yet
      About MollyMae
      Current: Forest Lake, Minnesota
      Birth: December 19
      On since: Apr 8, 2014
      I am currently a SAHM I work for a construction company 2 days a week but get to bring my son with me so that's wonderful. We enjoy being outside, four wheeling, boating, jet skiing. My husband and I have been married since 10/2013. Love being a momma!!!