Do you mean if the United States ceased to exist why would Florida not own the stuff in its own state?
The Soviet Union didn’t just vanish without a trace. The process of dissolution was complex and involved agreements among internal and international organizations. Russia was recognized under international law as the legal “successor” to the Soviet Union. That’s why Russia assumed the former Soviet seat as a permanent member of the UN Security Council. It’s also why Russia took on the responsibility for the Soviet Union’s debt obligations to other countries.