The Horse is “put down.” Why?

I don’t understand while horses that break legs are “put down” right on the track, in full view of the fans and the TV cameras. It happened Saturday at the Kentucky Derby. Runner-up Eight Belles had to be euthanized on the track after breaking both of her front ankles, marking the first time in the Derby’s 134-year history that a horse died. At least a tent could have covered the horse while all this was going on. And do you have to kill a lame horse? There is no way to set the bones in casts and let this philly live a quite life in someone’s pasture? Hey, this is 2008, yet modern veterinary medicine can’t respond.? I just don’t understand.