The showrunners knew they were adapting The Testaments, so they obviously needed Hannah for that. Scenes of Nick getting close to Hannah's "father" in Gilead were scrapped once that was coming out. Besides, I don't think it's supposed to be easy to just "go get" a child in Gilead living states away and get them across the border.

The show definitely did a switch-up on his character. It was just one of the confusing, baffling decisions this season, and trying to make sense of it all is a fool's errand.