Here's why:

* Lack of Nucleus: Prokaryotes have their genetic material (DNA) in a region called the nucleoid, but it's not enclosed within a membrane.

* No Membrane-Bound Organelles: They lack structures like mitochondria, Golgi apparatus, endoplasmic reticulum, lysosomes, etc., which are found in eukaryotic cells.

This difference in cellular structure is fundamental and defines the distinction between prokaryotes (bacteria and archaea) and eukaryotes (plants, animals, fungi, protists).