Here's why:

* Binary fission is a form of asexual reproduction where a single-celled organism divides into two identical daughter cells. This is the primary mode of reproduction for prokaryotes, which include bacteria and archaea.

* Mitosis is a form of cell division that occurs in eukaryotic cells (cells with a nucleus and other membrane-bound organelles). It involves the duplication of chromosomes and the division of the nucleus, followed by the division of the cytoplasm.

Key differences:

* Prokaryotes lack a nucleus and other membrane-bound organelles, while eukaryotes have them.

* Prokaryotes have a single circular chromosome, while eukaryotes have multiple linear chromosomes.

* Binary fission is a simpler process than mitosis, involving fewer steps and no spindle fibers.

Therefore, prokaryotes reproduce by binary fission, while eukaryotes reproduce by mitosis.