If I had to guess, this is probably a large part of it. They didn't have anything doing much better, it was stable, relatively cheap, and had a decently-rabid fanbase to keep its head just above water. The premise (such as it was, post-Season-5 where they just made up new threats every season) was flexible and perpetual.
IMO there was a lot of squandered potential in it, but by the time it ended it had essentially become a comfort-food show, and I suspect CW realized that was a niche they could milk as long as the actors didn't get bored or die.
I think it's telling that although it had a long run, neither of the spinoffs (Bloodlines, which was horrid; Wayward Sisters, which had a slew of amazing actresses ready to go) got the greenlight. So clearly CW wasn't willing to commit to branching out from Supernatural even while they kept the parent show running. I think they believed that most of the staying power was based around the main trio and not the show's lore and world-building; they were probably right.
