Crawl spaces are shallow structures installed directly into the dirt under your home. As such, they tend to soak up the natural cold of the earth around your home and transfer it into the air in your crawl space.
If you have utilities in your crawl space such as your water heater, furnace, or hot water pipes, your energy bills will suffer. The fluctuation in temperature will force your heating and air conditioning systems to work harder, causing your energy costs to rise.