One possible explanation for the fact that some simple, one celled organisms did not even evolve into complex multicellular organisms is that

The answer is that they have traits that enabled them to survive in a changing environment.

During the evolution of the Earth, and later during the evolution of life, the environment was not constant. Change in the environment could lead to an extinction of some species. But, some species were able to adapt or could tolerate a wide range of environmental factors. One of the explanation, why unicellular organisms did not evolve into complex multicellular organisms is because they have traits that help them to tolerate the wide range of factors, and thus, survive in changing environment.

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