It's a combination of a number of things, but the mechanically weirdness I explained earlier is the biggest factor. For those who aren't aware, the Mutant Clade's special thing is that they can swap out one of their Variations (aka their powers) for any Variation with half that dot rating or less. While they're restricted to the list of Universal Variations for the replacement power, this still gives them over 25 different powers they can give themselves at any given time (which gets more absurd when you realize that some Variations actually represent a wide number of different powers all squished together, such as Enhanced Attribute). This is just too flexible and fiddly to work in the quest format.
On top of that, I don't think Mutant has nearly as distinct an aesthetic as the other Clades, so you don't really lose much from me not offering that option. So there's a big, glaring reason for Mutant to not be an option, and no real compelling counterargument.
No, we're moving out of the character creation phase at this point and entering the meat of the quest. The next vote is going to be an actual choice you're making as Subject 37 rather than further defining who she is.
Speaking of the next chapter, it's taking me a bit longer than I'd anticipated. I'm still going to try to get it out this week, but don't be surprised if that doesn't happen. The good news is that this is because it's actually an eventful chapter in which things are happening rather than the fluffed up character creation of the past two chapters. Now that I have a main character rather than an undefined blob, I can actually have a plot!