Here's why:

* Prokaryotic cells are simple: They lack the complex internal organization found in eukaryotic cells, including membrane-bound organelles like mitochondria.

* Endosymbiotic theory: Mitochondria are thought to have originated from free-living bacteria that were engulfed by early eukaryotic cells. This process, known as endosymbiosis, led to the mitochondria becoming an integral part of the eukaryotic cell.

* Energy production: Prokaryotic cells have their own mechanisms for generating energy, typically through the cell membrane. They don't need mitochondria.

In summary, mitochondria are a defining characteristic of eukaryotic cells, and are not present in prokaryotic cells.