Here's why:

* Formation: Cumulonimbus clouds form from the rising and cooling of warm, moist air in the troposphere. This rising air creates the towering vertical structure of these clouds.

* Tropopause: The tropopause is the boundary between the troposphere and the stratosphere. It's a barrier to vertical air movement. Cumulonimbus clouds cannot penetrate the tropopause because the air in the stratosphere is too stable and dry.

So, while Cumulonimbus clouds are massive and can reach great heights, they are still confined to the troposphere.