I’m not an expert in this area (it’s more of a meteorology/geography type thing!), but I’ll explain what I know 🙂
As I understand it there are two factors:
1) This is the main one – Clouds are made up of REALLY small water droplets (between ~5-50 thousandths of a millimeter!) Really small, light things don’t fall very quickly as the resistance of the air they’re trying to fall through is very high because they are so small.
2) Clouds tend to form in areas where air rises, so there is a constant stream of upward moving air to keep them afloat!