I would Amore to know why te all got end the mostra after 15.I look inoltrare, avanti see te guys every week. Supernatural got other ways to go,they don't have to end.I hope te all leave a cliffhanger that way te can return. I think last one should be a 2 ora

They wanted to end the mostra on their own terms, rather than dragging it on and risking it becoming less popular. They didn't want it ending due to cancellation, particularly with the cliff-hanger endings the seasons usually had. Knowing that season 15 would be the final season enabled them to tie up as many loose ends as they could, bring back loads of earlier actors for cameos and reference precedente scenes. As to why they chose the 15th season as the last - they just felt 15 was a good round number to go out on.