Kun needed the energies of Lukes 20 something Jedi Academy to revive him.
Ragnos needed the energies of entire Dark Side nexuses to revive him, including the Great Massassi temple which amped said Jedi Academy to knock an imperial starfleet outside the Yavin system.
Ragnos being revived would require all of Lukes students circa 13 ABY and himseld included to stop him, though something similar can be made for Kun.
Pls no h8
Exar Kun corrupted Luke’s most powerful student, Kyp Durron, and attempted to resurrect himself by draining power from Luke and the other trainees.
- Jedi VS Sith The Essential Guide to the Force
Eagerly but cautiously, Kun observes each arrival, probing for weaknesses and the power he needs to restore his lost reserves of energy.With a nucleus of followers to provide him energy-providing anger and fear, Kun will have enough power to escape his exile and take on human form.
- Jedi Academy Sourcebook
Not that it's relevant, Kun performs better feats in his prime than he ever did in JA.