My naive thought - complexity kills all things. Eukaryotic cell is bigger, has more metabolic processes, can have specialized life cycle, etc. All of which mean

My naive thought - complexity kills all things. Eukaryotic cell is bigger, has more metabolic processes, can have specialized life cycle, etc. All of which means that it is slower growing and could be out-competed by the faster-growing, but less optimal organism. Natural selection only picks those who survived, not "the best".