Well, let's not forget that it wasn't always the case that pilots were aired. Had the show been picked up it's possible the pilot would have never aired, especially if it contradicted the series to follow.
Not in this case, because The Questor Tapes
was a "backdoor" pilot, designed to work as both a standalone TV movie and a pilot for an ongoing series. That way, if the series didn't go forward, the expense of the pilot would not be wasted, because it could still be aired and syndicated as a movie, which it was. So it was always going to be aired. This is how all of Roddenberry's '70s pilots were done. Other notable '70s series like The Six Million Dollar Man
and The Incredible Hulk
also began as TV movies (2-3 movies, in fact) before debuting as weekly series.
Indeed, TQT was
picked up as a series before the pilot aired, but it aired anyway. It fell through because Roddenberry had a falling out with Universal and NBC about the direction for the series. They wanted to drop Mike Farrell's character, and -- yes -- to disregard the ending of the movie. Roddenberry refused, so the deal fell through. They may have been right about the latter, but Roddenberry was right about the former: the show needed the Questor-Jerry relationship.