Race horse leg injury

Question
I don't understand why the leg fractures to Barbaro, the lead contender for the Triple Crown, MUST be life-threatening.  I understand why his racing career is over but I don't know why it's almost certain he will have to be euthanized.  The example I read said that a human w/ the same injury would have to be put to bed for 6 weeks, but that would be impossible for a horse.  Well, I don't know many people who've been required to do more than wear a cast and "stay off" the fracture as much as possible; why can't a horse wear a cast and maybe put in one of those hammock-type harnesses for part of each day or something like that while he recovers?  Would a horse simply not tolerate it?  Why is a leg fracture almost certain to be life-ending for a horse?

Answer
Good question.  The ability to maintain the cast/hammock-type harness suspension completely depends on the horse.  Some horses have calm natures and understand, some don't.  Nureyev handled it beautifully and continued with his highly influential stallion career.  Ruffian could not do it, however, and she was euthenized.  It just depends on the horse.  We will watch and see.  Thanks, Lora