Based on the way I was taught history in public schools, the big powers were:
-Egypt
-Babylonia
-Persia
-France
-England
-Germany (later)
-Rome
-Mongolia
-Zulu
-Carthage
-America
-Japan
-China
-Maya/Inca/Sioux/ShoShon/Aztec all were mentioned.
But Poland/Korea/Ethopia? Not even a footnote.
Some Civs are in the game for being major powers within their sphere of influence. Others are in the game simply for being interesting (culturally or historically).
The Zulus fit more into the latter. Their big claim to fame was "heavily resisting the Europeans despite the odds". Their war-friendly culture also factors into it.
Similarly, Carthage's big claim to fame was the Punic Wars, as they're remembered more for it than for their culture, or trading empire (though the latter is often referenced as being "Phoenician"). The Aztecs have shades of this as well, being remembered more for their unique culture than their Empire (the Incan empire has them beat in land, power and influence, but the Aztecs are more famous).
Poland was considered more due to their large Polish fanbase, but, much like with Austria and Sweden, Poland's history is rich enough to make an interesting Civ.
Korea was always a unique culture and state(s), and relevant to Eastern Asian history. Other games (Age of Empire, Rise of Nations, etc.) have also included them.
Ethiopia, as was mentioned, is known for having been a major African power for most of its existence (though they lost a lot of influence after 1974), resisting colonization being one of its most notable achievements.
After including the major powers, the developers mostly turned to options that looked interesting and/or fun (see: Polynesia, Celts, Huns).