As I recall Bob Smith had left the GOP, too, so that made sense--he wasn't Republican. But what is their rule when there IS no incumbent?
Before the primary, Bob Smith had re-joined the GOP. But nonetheless, the NRSC sat it out - which was quite unusual for them.
Regarding an open seat, the NRSC usually doesn't pick sides. There have been a few exceptions over the years - but usually they leave it up to the voters to decide.