Why are deserts generally colder at night than other areas are?

The wide fluctuation in temperatures in most deserts between day and night is due to the lack of a large body of water. Large bodies of water absorb sunlight all day but can hold more heat than the land. When the sun goes down the water is still radiating the heat it absorbed all day making the air around it warmer. The ground radiates heat from the sunlight all day elevating the temperatures in the daytime. When the sun goes down in the desert there is no body of water to keep radiating heat, so the temperature drops more than in a coastal area.

Actually,

A desert has little moisture in the air and few clouds. These two act as blankets to hold in the heat at night. Without a blanket, the heat absorbed by the sand, soil and rocks of the desert is radiated back into space and the temperature drops at night.