Why are protists no longer a kingdom?

Because Protist has many organisms that are related to the other kingdoms of animals, plants, and fungi. Protists is a word that is know used as a "eukaryote that isn't a plant, animal, or fungus."

Protists are no longer classified as a kingdom because advancements in molecular biology and genetic analysis have shown that they are a diverse group of organisms that do not share enough common characteristics to be grouped together. Instead, protists are now classified into multiple kingdoms based on their evolutionary relationships and genetic differences.

Protists are no longer classified as a single kingdom because they are a highly diverse group of organisms that do not share enough common characteristics to be grouped together in a meaningful way. Advances in molecular biology and phylogenetics have revealed that protists are not a monophyletic group, meaning they do not share a common ancestor to the exclusion of other organisms.

Instead, protists are spread across multiple branches of the evolutionary tree. Some protists are more closely related to plants, others to animals, and still others to fungi. As a result, the old classification of protists as a kingdom has been replaced by more specific groupings based on evolutionary relationships and genetic similarities.

Today, organisms that were once considered protists are classified into various kingdoms, such as the Kingdom Plantae, Kingdom Animalia, Kingdom Fungi, and several others based on their distinct characteristics and evolutionary history.