Here's why:

* The troposphere is the lowest layer of the atmosphere. It's where we live, and where weather happens.

* Clouds form when water vapor in the air condenses into tiny water droplets or ice crystals. This condensation happens when the air cools and can no longer hold as much moisture.

* The troposphere is the warmest layer near the surface and cools with altitude. As warm, moist air rises, it cools and condenses, forming clouds.

While clouds can form in other atmospheric layers (like the stratosphere), they are much less common due to the drier and colder conditions.

Let me know if you have more questions about the atmosphere or clouds!