Why are bacteria placed in a kingdom by themselves?

Answer 1

Bacteria are prokaryotes, whose cells have no nuclei, and they are much more like other bacteria than like any other organisms.

Answer 2

Bacteria are placed in a kingdom by themselves, called Monera, because they are prokaryotic organisms with unique characteristics that separate them from other organisms with nuclei. They have a simple cellular structure lacking membrane-bound organelles and a true nucleus, making them distinct from eukaryotic organisms.