Here's why:

* Stratus clouds are low-lying clouds that form in a sheet-like layer. They are typically gray and cover the entire sky, blocking out the sun and giving the appearance of a flat, uniform gray.

* Other cloud types like cumulus, cirrus, or cumulonimbus usually have distinct shapes and don't cover the entire sky in a uniform way.

Here's a simple analogy: Imagine a blanket spread out over the entire sky - that's what a stratus cloud looks like!