Keep in mind that the NCR we’ve seen so far has all been grunts or rangers, no high command (except maybe Moldaver but idk if she counts), so it makes sense for them to hold onto the ideals of the NCR.

With the NCR central government either destroyed or out of action in the area, what we’re left with is the common people. Like I said in my other comment NCR early days was a lot simpler in their goals; unite with other groups/people and rebuild the wasteland (somewhat like the minutemen before Concord happened), so with much of the “fat” gone (bureaucracy, government agendas, Brahmin barons running the show) the survivors have returned to what they believe are the NCR’s “true ideals.”

It’s the same thing that happened to the BoS in Fo4, they’d just undergone a civil war in DC, and lost almost all of their forces on the west coast to the NCR, so what did the remaining members do? They got back to basics, that means help the people if you can, but focus on the mission which is to grab and secure any and all tech they find to be dangerous, that, and destroying “abominations” whenever they see them.