This is what everyone is talking about: https://en.m.wikipedia.org/wiki/Serbia_and_Montenegro_in_the_Eurovision_Song_Contest
Serbia and Montenegro used to be a country. It's like how there used to be Czechoslovakia. If Czechoslovakia had participated and someone said "you missed Czechoslovakia" and then you said "the Czech Republic and Slovakia are both there", that wouldn't answer their question.
I think the only reason only 42% of people get Serbia and Montenegro is because that either people don't know of any of those countries, didn't know they were together at one point, or knew and just thought there name was Serbia or Yugoslavia.
See also List of countries in the Eurovision Song Contest: Fifty-two countries have participated in the Eurovision Song Contest since it started in 1956.
Serbia and Montenegro used to be a country. It's like how there used to be Czechoslovakia. If Czechoslovakia had participated and someone said "you missed Czechoslovakia" and then you said "the Czech Republic and Slovakia are both there", that wouldn't answer their question.