You've been trying to say that for the better part of the thread.
I understand that.
You don't understand what I'm telling you why it isn't an explanation.
ok...
why isn't it a standard trope of SciFi that the aliens speak with that accent?
because the Cold War did not end in the 1950's... or the 1960's... or the 1970's... one can say that the Apollo Mission was part of the cold war. Hollywood both influences and reflects public opinion. Public opinion during that time frame, and for decades thereafter, was that "Slavic" sounding dialects came from
"Rooskies", "Comrades", and other "unAmerican Commie Bastard shitholes".
Really, it is quite simple.. Like asking why the casting of Nichelle Nichols as Uhura on Star Trek was "such a big deal".
As for why HE was selected to speak the greeting, it was because he was the Secretary General of the UN at the time, and his voice spoke for all nations, regardless of Hollywood influence... cause that was politics, not the movies.
You asked why him on the record and why not more of him in SciFi. I answered both.