Numerous reasons. The nature of the two wars is completely different. The U.S. never attacked the Chinese mainland, unlike the Japanese during WWII. The Japanese were trying to take over China because they saw it as a part of their sphere of influence, while the UN had a completely different mandate it was enforcing. The Chinese never declared war, instead they sent "volunteers" to help the North Koreans. One last reason is that Japan was particularly brutal in China and never atoned for their crimes to the satisfaction of many Chinese. China was devastated by war with Japan from 1931-1945, and also had prior conflicts with Japan (Taiwan/Formosa, Korea, Port Arthur come to mind). The Korean War didn't even last 3 full years for the Chinese. Not enough time to have it be part of the cultural memory of a nation.

Edit: Sources worth checking out related to this.

China and Japan: History, Trends, and Prospects by Christopher Howe, Oxford UP

Propaganda and Culture in Mao's China: Deng Tuo and the Intelligentsia, Timothy Creek, Oxford UP

The Chinese view, with some justification, the Korean war as a victory. Barely a year old and still recovering from their post world and civil war devastation, they managed to keep a superpower and the majority of the UN powers in check. It's an amazing military feat.

Source: Oliver Stein, The Demise and Rise of China