Here's why:

* Protists are a diverse group of eukaryotic organisms (meaning they have a nucleus and other membrane-bound organelles).

* Many protists are unicellular, while others are multicellular or form colonies.

* Examples of single-celled protists include amoeba, paramecium, and diatoms.

While there are some single-celled organisms in other kingdoms, like bacteria (Kingdom Bacteria) and archaea (Kingdom Archaea), the vast majority of single-celled organisms are classified as protists.