For one thing, it endangers the shape of the plot. Everything has to build up to the climax. If this is not the climax -- and even in the denouement, a change will mean this is not the true climax; you can't have true story after it, only tying up loose ends -- a lot of events have to be repointing, so they shoot toward the real climax, not this one. The erstwhile climax will, no doubt, send the entire story slinging off on a new path, which helps keep the reader awake, but it will need redesign for that, and the tension level has to rise after it, so the plot twist needs to make things more dangerous, not dissipate tension in an explosion. (Though usually the character's act does help reformat it for that.) Even if the event could work as a sequel hook, sequels can't be conjured up in the last pages; they have to be set up in the middle of the work.
For another, it means the story will be longer. If it had flopped in the middle of the Unpublishable Void, that might be a good thing, but if it might have been salable as a short work -- not so good.
And then there's the sensation of heading back down into the Valley Full of Clouds, trying to blaze a trail for the story. . . the mere fact that it looked like the climax while down among the trees means that it's harder to figure out what happens next. Inspiration must be wooed.