I would assume because the Soviet Union had a recognised successor state (being the Russian Federation), where as Yugoslavia did not have a recognised successor state.
The former Yugoslavia had five legally recognized successor states.
https://treaties.un.org/doc/Treaties/2001/06/20010629%2001-3...
The former Yugoslavia had five legally recognized successor states.
https://treaties.un.org/doc/Treaties/2001/06/20010629%2001-3...