The “why” is explained on pontosnews.gr by the educator of Pontic origin, Anna Kalaitzidou.
professor of Russian language.
One of you, Evi sent us this very interesting article!

Very often we hear the words “astronaut” and “cosmonaut” and consider them synonymous. Are they, though? They are not.
The historical review of the origin of the words gives a first idea about their difference.
The word “astronaut” is a compound word and consists of “astron” (star) and “nautes” (sailor). Therefore, an astronaut is a specially trained person who explores the stars.

For the first time, the word astronaut was used by the American author Neil Ronald Jones in the short story titled “The Death’s Head Meteor,” in 1930. Generally, the term “astronaut” preceded the term “cosmonaut” thanks to science fiction books that flooded and continue to flood the American market.
Conversely, the word “cosmonaut” comes from the words “kosmos” and “nautes,” where “kosmos” in ancient Greek means “ornament,” and in modern Greek has come to mean “the universe.” Therefore, the concept of “cosmonaut” is broader than the term “astronaut” and implies the specially trained person who explores the universe in its entirety.
However, these two terms not only have this difference but also have a political nuance.
In the USA and in countries that use the space stations and spacecraft of the Americans, the term “astronaut” is attributed to all those who begin the required training to become “space travelers,” regardless of whether they ultimately do so or not.
In the USA, it is characteristic that “the astronaut badge” is awarded to those who travel to an altitude greater than 50 miles, i.e., 80.5 kilometers. Conversely, in the former Soviet Union and in today’s Russia, anyone who makes a space journey from a space station on Russian territory and completes it is called a “cosmonaut.”
Therefore, one cannot be considered a cosmonaut if they have been trained but has not completed a space journey.
Σε ευχαριστούμε Εύη!!!