I'm not sure why he did that because I can see ISTJ or ESTJ as utilitarian as well.I agree regarding Keirsey's descriptions seeming to favor N. Here's a current site that gives some illustration of his divisions - https://keirsey.com/temperament-overview/
He posits SPs and NTs as utilitarian - focused on doing what works - and SJs and NFs as cooperative - focused on doing what's right. It's an interesting observation but I don't personally always think it's correct, or that it does a better job at dividing up than ST/SF/NT/NF. I've heard before that Keirsey drew SJ/SP/NF/NT from the 4 classic temperaments, but I don't know how true that is.
From Please Understand Me II, I vaguely remember him labeling guardians as melancholics, artisans as sanguines, idealists as cholerics, and rationals as phlegmatics. I don't really buy into that either.
I think he's trying to make his products and services marketable and in order to do so he wants to make the types fit into neat little boxes and labels. I checked his site out and apparently under self-image of the four temperaments 'willpower' is associated with NT rationals but I wouldn't even say that's right because out of all of the types I'd associate willpower with ESTPs. I'll even say Keirsey is responsible for a lot of the S stereotypes and negativity, when you promote labels such as ISTJ "Inspector" compared to INTJ "Mastermind" and ESTP "Promoter" compared to ENTP "Inventor" it's pretty easy to see why there's a divide concerning N and S.