In the older three-kingdom system of classifying living organisms, there were animals, plants (including fungi) and Protista (all others).
In this system, Protista would be the most diverse group because it includes the innumerable and ubiquitous unicellular, multicellular and colonial organisms that live in water or soil, and on the skin of animals. Included would be bacteria ,of which there are thousands of varieties, not to mention the many forms of protozoans.
In the modern classification, there are six kingdom divisions as follows:
5. Modern bacteria
6. Prokaryotic bacteria
Thus, Protista, in essence, is a catchall category for the diverse living organisms that are not categorized as animals, plants or fungi. Members of kingdoms other than Protista are subject to the limitations of each category, which would include method of reproduction. Protista do not have such limitations.