The best answer I can give is that they did- indirectly. Essentially, the Soviet Union did not want to engage directly in a ground war with the United States, especially since such a war would have escalated into a real war. Instead, they engaged in a proxy war. Russian weapons and resources flowed into China which in turn flowed them into Korea.

Anyways, the USSR wanted the US to waste resources by fighting in a pointless war. The USSR did not want to lose men themselves or risk going into a direct war with the United States since that would ultimately end in mutually assured destruction. The Russians took a similar kind of stance in the Vietnam war as well, only offering weapons to the NVA.

The US would later do the same thing in Afghanistan, offering weapons to extremists to fight the Soviet occupation in hopes of sparking a proxy war.