BobCo mentioned the crux of the matter : Low percentage.

Shooting straight down a difficult shot to make. That lowers your percentage off of the bat. Then add the fact that your 9 or 10 inch kill zone shrinks to about 4 or 5 inches, that lowers your percentage by another 50%.

IF you hit the center of the spine, yes you will probably drop the deer right there, but the odds are against you. Miss the spine either side and you've probably got a single lung and no heart hit at best . You may also catch a rib at the terminus where it's strongest rather than in it's middle where it's weakest.

In the spirit of full disclosure, I myself would be tempted to take such a shot, but then I'd lean out and look down and probably get a bit of vertigo and have to forget it