Generally they are made of plywood. Sometimes, pressure treated plywood. Once in a while someone will make one out of 2x4's and plywood.
Don't give it a lot of thought, the temperature and humidity difference does not seem to make much difference.
I guess I'm somewhat of an expert on crawlspace doors because I am a home inspector, and go into crawlspaces on a daily basis.
The damage I see is usually:
1. Loose or broken hinges.
2. Wood rot because the door was in contact with the ground (or mulch).
3. Age, and the weather finally got to it.