Why you have to put a horse down

Sometimes the subreddit "Explain it like I'm 5" has a really interesting question like: "Why do horses need to be put down when they break a leg while most other animals don't?"  There's a lot here but, in a nutshell, it comes down the fact that horses bones are very dense and do not heal well.  Also, horses do not respond well to palliative measures and if they're not standing up, there's all sorts of bad ancillary effects.