Some shows, like Agents of S.H.I.E.L.D. in Season 5 and Castle for what ultimately did prove to be its final season, craft season finales with wiggle room just in case a late-in-the-game cancellation happens.
Basically, series are best-served by having advance notice of the coming end, and Supernatural has more than a full season to build to a grand finale.
Otherwise, a show might end a season on a killer cliffhanger, only to be cancelled and leave fans hanging with more questions than answers, as was the case with Lucifer before it was saved by Netflix.