Why did some organisms never evolve beyond single cells?

The answer

Single-celled organisms like bacteria have thrived for over 3.5 billion years because simplicity offers enormous advantages: rapid reproduction, quick adaptation to environmental changes, and minimal energy requirements. Multicellularity evolved independently at least 25 times, but it requires costly cooperation between cells. Bacteria remain the most abundant life form on Earth, outnumbering all multicellular organisms combined, proving that evolutionary 'progress' is not always toward complexity.