They needed to free up manufacturing capacity for the EV6 and other better selling models, and the Stinger was by far the lowest hanging fruit. It was the worst selling Kia every year of its production. I'm curious why they even brought it to market. The ICE sedan segment had been shrinking in favor of crossovers and SUV's long before they revealed the concept.
I'm guessing they wanted to give their brand a "performance" edge so they started developing the Stinger, but 3/4 of the way through realized electric was where the performance segment was heading. So they released the Stinger anyway to recoup some of their investment, knowing full well the electric crossover EV6 would soon replace it as their performance flagship. Explains why they treated the Stinger as an afterthought after releasing it.
I just wanted a growly V6 before they were outlawed, and the Stinger ticked all the boxes and then some.