Here's why:

* Domains are the broadest classifications in the biological hierarchy. There are three recognized domains: Bacteria, Archaea, and Eukarya.

* Kingdoms are within domains. For example, the Domain Eukarya contains the Kingdoms Animalia, Plantae, Fungi, and Protista.

* Phyla are within Kingdoms.

* Classes are within Phyla.

* Orders are within Classes.

* Families are within Orders.

* Genera are within Families.

* Species are the most specific level of classification.

Since Domains encompass all the other ranks, they contain the largest number of organisms.