That makes a lot of sense.
Becoming a coordinator at the NFL level is the pinnacle of football strategy and tactics. A coordinator can immerse himself in the Xs and Os, getting the control and flexibility to be innovative and creative while having all or most of the say in play calling.
Head coach is a totally different animal. It’s about the press, deciding what hotel you’re going to stay at, personalities, dealing with ownership and front office, and on and on and on. A lot of it ends up being CEO / people management stuff that only tangentially involves football. Frankly, guys who are really, really good at Xs and Os stuff seem to rarely have this other skill set. It’s why the Norv Turners and Wade Phillipses of the world eventually realized they’re happier as coordinators.