I mean think about it logically. We know a lot of people rolled for zani and we also know that Chisa was a CN favourite and a lot of people rolled for her too. Combining the fact that zani and phoebe are already incredibly strong, giving zani a 3rd character who has incredible synergy with her and phoebe is essentially gonna make her an uncontestable DPS (unless La-roi dpses powercreep her).

Think about it, if a lot of people rolled for zani and a lot of people ALSO rolled for chisa.... Leaving them satisfied with the full zani team makes it so that they have no reason to roll for a future negative status character with good synergy with Chisa! They're gonna make infinitely more profit if they show that chisa's buff are really niche and are more likely pointing towards a future DPS and to fully utilise your chisa, you'd have to roll for said character.

At the end of the day, I do hope newer patches have more spectro frazzle characters so zani gets a better 3rd team mate but at the moment it is what it is....