Why is the kingdom protista considered a catch all kingdom?

The Kingdom Protista is a kingdom composed of single-celled organisms (e.g. protozoa, algae, fungi, slime molds). Aside from being called as the catchall kingdom, single-celled organisms are also called the left-over kingdom. This is because when scientists have a hard-time classifying eukaryotic cells they categorize them under this kingdom.

The Protista kingdom is composed of heterogeneous species that can perform the same functions as the other kingdoms do but they lack some membranes and certain functions in their cells to verify their classifications.
An example of a Protista is the algae. It first belonged to the plant kingdom but they lack the differentiated tissues of mosses, ferns, and seed plants that is why they were reclassified by scientists. Slime molds were also first classified in the fungi kingdom but as the information on its process accumulated they were group under the Protistas.