Why protista has not any well-defined boundaries
Answers
the boundaries of this kingdom are not well defined. Members of Protista are primarily aquatic. Being eukaryotes, the protistan cell body contains a well-defined nucleus and other membrane-bound organelles.
Protista are aquatic organisms, that do not have well defined boundaries because the organism which belongs to protista are more relevant to other groups other than protista.
Organisms which are categorised under protista group have more similar functions, like body organelles, feeding habit, etc with organisms of other group rather than within the same group. It makes it more difficult to categorise and put a well-defined boundaries to this group.
Most of the organisms in protista aquatic and have well defined nucleus and other body organelles, which is common to the the organisms of other group. Also these protista organisms are eukaryotic in nature, therefore it is more advanced then prokaryotic organisms.