Are the 10 different Zerg going to each have their own motivations for sticking with Kerrigan? I highly doubt that those motivations are going to be any more nuanced than "they got infested." [Nicol's proposed] on-the-job conflicts are absolutely inconsequential. Compare that to the situation aboard the Hyperion... Matt Horner's trying to play the good guy all these four years while Jim's making love to the bottle, suddenly Tychus and Gabe come aboard and are able to exert all this influence over Jimmy. 10 missions into this campaign, Matt Horner says "When you told me how Mengsk betrayed you for personal gain, I felt for you. But now I really know what that must have been like. I'm outta here." Up and leaves. That's it, no more Matt Horner on the Hyperion.
Sounds like an emotional turning point for Raynor, right? A turning point achieved through the execution of Matt Horner's free will as a character. Just the sort of thing Zerg characters are unable to do. If Stukov or Ethan Stewart are any indication, free will -- the sort of free will that Kerrigan is oh-so-known for exhibiting -- just isn't an option for other Zerg.