In Qing JPN war, JPN was not fighting against Korea. I am afraid Korea was too weak to be an enemy of JPN. JPN was busy fighting a war against great powers, China, Russia, and US.

In the case of Qing JPN war, the difference was modernization and nationalism. Qing was a Manchurian dynasty, so the Han Chinese were not enthusiastic about the war. After the advent of Chinese nationalism, the war against China became a war without an end.

Regarding Russo JPN war, there are two reasons. First, JPN had a military alliance with UK. UK did not send their military, but their information and support were crucial. The other thing was the battle in the sea of JPN. Togo Heihachiro destroyed almost all Russian Baltic fleets. This sea battle was one of the extremely one-sided victories. Without their fleets, Russians could not invade JPN, therefore, they chose to end the war.