With crassulacean acid metabolism, why is carbon dioxide only gathered at night?

Crassulacean acid metabolism (CAM) is an elaboration of the typical plant photosynthetic pathway that endows plants with remarkable water use efficiency. Through the use of a carbon-concentrating mechanism, plants that employ CAM are able to fix carbon while conserving water by minimizing transpiration during the hottest part of the day, or during stressful conditions.

...uptake of CO2 during the CAM cycle occurs primarily at night. This often means that CAM plants open their stomata for gas exchange at night when the temperature is lower and the relative humidity is higher — significantly reducing the rate of water loss due to evapotranspiration.