Xeno got idea.
We all know that the PPG and the RRB started off as enemies. It wouldn't be too far out in the realm of possibility to make them have a rocky relationship (Coming off the enemy stage and going into the tentative friend stage), where they're starting to see each other how they should.
Dormant for a few years, gathering strength, Him decided to strike at the very heart of the Powerpuff Girls! Their HEARTS! Using his diabolicle Underworld Magics, he mixes six potions, potions he distributes to both the PPG and the RRB, in hopes of making them fall so madly in love with each other, they'll both be so preoccupied by affection, they'll ignore most anything else, thus allowing HIm ample apportunity to crush to world in his oppressive claw.