Failed pilots never air in the US unless they are ordered as a mini/back door pilot.
Well, "rarely" rather than "never." At least, if it's "never" these days, it wasn't always. Networks used to burn off failed pilots as "specials" or "showcases." But that was back when networks still showed things like specials and movies of the week. Network programming seems to have become narrower in focus these days.
Mr. Laser Beam wrote:
Speaking of failed pilots: Did Doorways ever air?
Do you mean the George R. R. Martin alternate-timelines pilot that came along a year or two before Sliders
? As far as I know, no, it never aired.