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.
Thou hast well said, Yugoslavia has no successor states: For Yugo hast had five successor states; and the domain thou now hast is not thy successor state: in that saidst thou truly.
If I'm not mistaken, and please correct me if I am, the last republic to leave the USSR was Kazakhstan - making that nation the actual USSR successor state. Though the capital was in Moscow, Russia left the USSR while the USSR still existed, and thus is not the USSR successor state.
Legally Russia is the internationally recognized successor state. Russia even paid off the whole Soviet debt, but in exchange inherited all of the USSR's legal privileges (right to have nukes under NPT, right to the spot on the Sec Council, right to observer state in Danube, etc)
https://treaties.un.org/doc/Treaties/2001/06/20010629%2001-3...