Low-pressure systems form clouds because they are associated with rising air.
Explanation:
With the passage of a low-pressure system we can almost always expect rain and certainly expect cloudiness to increase. This is because in the center of the system is the point of lowest pressure which is associated with air rising away from the center. As air rises, it cools and forms cloud when the condensation point is reached.
This is the opposite of what happens in high-pressure systems since these are characterized by air that is sinking towards the center of the high-pressure system.
