Final answer:
Protists and bacteria are grouped into different domains because they belong to different types of organisms with distinct characteristics.
Step-by-step explanation:
Protists and bacteria are grouped into different domains because they belong to different types of organisms with distinct characteristics. In the three-domain system, the domain Bacteria includes all organisms in the kingdom Bacteria, while the domain Eukarya includes protists, among other eukaryotes. This grouping is based on differences in cell structure, membrane composition, and genetic characteristics.