Why does ernst mayr's biological species concept not apply to archaea and bacteria?

This is because Bacteria and Archaea don't reproduce sexually.

Ernst Mayr defined a species as:

"species are groups of interbreeding natural populations that are reproductively isolated from other such groups."

Bacteria reproduce through binary fission, a form of asexual reproduction.

Archaea also reproduce asexually by binary fission, fragmentation, or budding.