Final answer:
Kingdom Protista was abandoned due to the discovery of diverse evolutionary relationships among its members, with many protists more closely related to other kingdoms. This led to a reclassification based on monophyletic groups and the term 'protist' is now only informally used.
Step-by-step explanation:
The group traditionally known as Kingdom Protista was dissolved because molecular genetics revealed that the protists are too diverse, demonstrating that some are more closely related to animals, plants, or fungi than they are to other protists. This makes Protista a paraphyletic group, which does not include all descendants of its common ancestor, thereby conflicting with the current criteria for monophyletic groups in taxonomy. With advancements in phylogenetic analysis, instead of having one large, inclusive group, many individual groups have emerged, showing evolutionary relationships more precisely. Consequently, biologists have reclassified protists into new kingdoms or other existing kingdoms, leading to a more accurate reflection of their genetic and evolutionary relationships. The term 'protist' is still informally used to refer to a diverse group of eukaryotes.