>>509399803Horse legs have very poor healing capacity, mostly due to the extremity being 99% skin and bones.
Horse also have *extremely* poor capacity to not try to walk on an injured limb, due to evolution making "run the fuck away" their entire survival strategy. Most animals will limp or stay immobile if their leg hurt, horses will power through the pain just to stay mobile.
Combine the two and you have a patient for which a broken legs is pretty much a guaranteed death sentence from the fracture constantly worsening and sooner or later causing too much blood loss, maddening pain, global infection, or all of the above.
Racing horse owner being generally piece of shit who only care about money, they won't even bother trying and just get it euthanized on the spot unless the injury is very minor.