Why are protists, plants, fungi, and animals classified into the same domain but into different kingdoms ?

Archaea and Eubacteria are both prokaryotes. They can be separated mainly by their habitats. Why are protists, plants, fungi, and animals classifies into the same domain but into different kingdoms? They are the same domain because they are all Eukaryotes.

They are the same domain because they are all Eukaryotes. They are different from each other because they are not all Multicellular.

in 1859, charles darwin set out his theory of evolution by natural selection as an explanation for adaptation and speciation. he defined natural selection as the "principle by which each slight variation [of a trait], if useful, is preserved".