Why does biologists classified all species of prokaryotes..

[kimikoh]

Why does biologist classified all species of prokaryotes in a single kingdom called Monera?

[mith]

because they are similar.

[Darby]

They don't. Monera is a convenience - folks who work in the field tend to split it more than that. Even now, I'm teaching in beginning classes two prokaryote Kingdoms - Monera and Archaea - but the distinction is a bit technical for that level.

[MrMistery]

you should teach them as Bacteria and Archea. Monera is the old kingdom that included everything without the nucleus. You could say something as "similarities in rRNA have divided the old Monera kingdom into Bacteria and Archea. Archea, though prokaryotes, are more closely related to EKs than Bacteria".
Unfortunately in my country even university courses still use Whittaker's five kingdom system.

[Darby]

I think we're following the convention (for the moment) of leaving the original name, Monera, after the split-off. I'm also finding that Archaea are often being called bacteria in discussions, so the term, to some people at least, still applies to both groups (but I still call protozoans animals, so I'm not sure I've got a point here).

[MichaelXY]

Animalia is multicellular.
Just curious, what are you teaching, and what age group.
I think being a teacher is cool

[Darby]

I'm teaching college introductory courses.

And although Animalia is multicelled, the word animal is colloquial and understood, even applied to protozoans. It's even weirder with plants, now that the algae were kicked out.

Plus, if you're going to be a biologist, it's useful to be able to slide back and forth between conversational and technical language.