Think you wanna take a look at
Guilty Gear's Jack-O for how Kerrigan (or any other RTS commander) could work. The summoning units thing is going to be limited by neccessity, since there's no cursor or easy way to create macro-groups on a controller for quick micromanagement in a fighting game. Especially not since you need to be able to move the main fighter when needed. As such, one can perhaps build a Kerrigan by having the Zerglings etc. be semi-autonomous but directed with relatively simple instructions like Jack-O.
Can only guess, but maybe it's due to him having trouble imagining how to properly scale them? It doesn't hold as much water post-Ridley and we know it's possible (sure
has some unsolved jank and is undertuned in some areas, but his character works pretty much).
But, Bowser Jr.'s model seems to be pretty damn big (with
a model size comparable to and
). So larger mechs than Jr.'s may introduce problems relating to how to handle large hurtboxes, especially if the mechs are on the wider side compared to the rest of the cast. The wider (for a lack of a better term) a character is, the more the character risks getting hit by random **** suddenly. Whether it's an opponent's attack or suddenly appearing Bomb-Omb (
's tail hurtbox, for example).
But at the same time, it wouldn't feel realistic at all to have say Eggman be smaller than, y'know, Bowser Jr. So it appears to be a weird balancing act.
Edit: It's really difficult to say what's "too big", but there is a point where some start to feel that either the attacks "shouldn't hit" (Mewtwo's tail) or some people start to wish the character's survivability was buffed to compensate for the huge size and the resulting risk of getting hit (Ridley's weight). OTOH, an overtuned tank could be really problematic to play against, so Sakurai and co. probably want to avoid that too. This is the kind of weird balancing act a more conventional mech user may run into in the design phase is what I'm saying.
Yes, this is not direct evidence and yes, I'll happily eat crow if Eggman or another mech user turns out to be CP11. So in the end, this is just speculation based on circumstancial evidence - it's really difficult to prove why something has not happened as often as one might expect.