From what I understand, there isn't actually a reason, other than familiarity with the Zyuranger suits. I have heard some stupid reasons, including that there wasn't a black ranger on the Dairanger team, but no real reasons. I think it was mostly because they didn't want to use the villains from Dairanger, and therefore didn't use the Sentai footage from Dairanger. Which would've made alot more sense if they didn't decide "No, we can;t use Dairanger footage, but lets use footage of their six ranger anyways". Personally, I think it was a stupid idea not to use Dairanger footage, especially when Saban themselves knew that Power Rangers is a pretty poor show, and if the villains were a problem they just could've filmed new villain footage, but kept all the monsters that weren't, you know, bondage themed, but hey, to late to change it now.