When used as a theatrical effect to simulate fog carbon dioxide gas tends to stay near the floor of the stage. what might account for this effect?

because the carbon dioxide gas is denser than the air.

Step-by-step explanation:

This happens because the carbon dioxide gas is way denser than the air, as you may know, density is equal to mass/volume, so the fact that the carbon dioxide is denser means that a fixed volume of the gas is heavier than the air, and this is why the carbon dioxide "falls down"

Another similar example of this is the helium-filled balloons, as the helium is less dense than the air, the balloon tends to go upwards.