Answer:
Because of their mass
Step-by-step explanation:
Clouds are made of water right ? so you are asking why they don't fall down if air is lighter than water .
The two biggest reasons the clouds stay in the sky are
1. Small Drops
2. Wind
Small drops of water fall more slowly than big drops, as the drops of the clouds fall thought the air, the air pushes back to them, because Small drops has less mass than large drops, clouds can only contain small drops of water.