1. People REALLY liked him after his first appearance.
2. Now that we're officially half way through the final season, it's time to ramp things up to the climax. That means last episode establishing the army that wants to take out Aku once and for all. And as for Scaramoosh, it's time to let grumpy Aku finally know that the Samurai is still out there, and he doesn't have his sword, thus finally awakening the true evil inside Aku, and getting the ball REALLY rolling for the grand finale.
I mean Gennedy now has 5 episodes to basically get Jack out of his funk, make Ashi his protege, meet up with the Rebel army, finally Kill Aku once and for all, hopefully get back to the past, and maybe kill him there, and FINALLY have a satisfying epilogue to conclude the ENTIRE SERIES.