Question

what might be a reason for classifying these domains into a single kingdom

Answer 1

The question might mean something like this, although this only my assumption.

There is kingdom bacteria/archea. These generally fall together, although some will classify this as two different kingdoms, and could widely be referred to as the simplest kingdom available. This contains only prokaryotic organisms. A collective term for this would be kingdom monera, but thats a much older term that is rarely seen anymore.

The next, more advanced kingdom, would contain your protists. These are your single celled eukaryotic organisms. Not many of these exist today, which is why you will often see people fading away from the term protists. Examples of these though would be be your Amoebozoa and Excavata.

I dont really like where the question leads from this point onwards because you'll see as you get into later biology classes that this isn't necessarily correct. In order of complexity, however, your next most complex kingdom would be kingdom fungi. These contain both your prokaryotic and your eukaryotic yeast bsaed organisms, mycellia based organisms, and a wide variety of others. The reason that I say this isn't necessarily correct on this part and the next two kingdoms is because there are certain classes of organisms that fall within these three kingdoms that can be more complex than a future kingdom or less complex than a previous kingdom.

Regardless, following this pattern the next kingdom you would see is order of complexity is kingdom plantae. This is pretty self explanatory. These contain your basic photosynthetic organisms. This gets into the topic of complexity again though, there are actually a wide variety of fungi that are photosynthetic, so it's hard to say. Every organism that has been discovered to this date that falls within this kingdom however, has been discovered to belong to domain eukarya (eukaryotes).

Lasty you would have Kingdom Animala. These are your basic mammary gland containing organisms. Pretty easy to think of examples: humans, apes, etc... I don't do research on kingdom animalia but these are technically the most complex set of organisms. The introduction of an organ system and various other metabollic pathways begins with this kingdom.

I think complexity, in regards to this question, is referring to the overall structure (internal and external) of an organism. In this case, the above order you see would technically be correct. If you study an evolutionary timeline you would see the outward appearance of more complex features from kingdom to kingdom.

Hope this helps some, good luck!