It's because there's a little Naihanchi in every kata!
Every time we step from one technique to the next in any kata, we're doing Naihanchi. It doesn't matter if we're practicing the most "basic" Fukyugata Ichi or the most "advanced" Kusanku. Every time we take a step, we're doing Naihanchi. In Kishaba Juku, Naihanchi is our foundational kata; our root. If we do good Naihanchi, then all the other kata will also become good.