Why have scientists recently rejected classifying protists in their own kingdom?

Biologists used to classify all protists as part of Kingdom Protista, but almost every rule used to describe them has some major exception. They are now attempting to revise the classification of the organisms in this kingdom. Some scientists have tried to sort protists in comparison to three of the other kingdoms: plant, animal and fungus. This has not worked because many protists exhibit characteristics from two or more of these kingdoms. The current attempts of classification focus on grouping protists into between three and ten kingdoms, based on species' common evolutionary ancestors.