If a horse breaks its legs, why does it need to be put down?

This has nothing to do with people being "in it for themselves." Even someone completely ignorant to horses and racing had to catch a clip about Barbaro at some point during his attempted recovery and know what his owners were willing to do to try and save him. But the above poster is right; horses are exceptionally complicated where leg injuries are concerned. They need to be able to stand; try telling a 1400 pound animal that's used to getting out and running to hold still for a few more months while she's poked and prodded and in pain. More often than not, trying to save a horse with the kind of injuries Eight Belles sustained winds up putting the animal through needless amounts of suffering before they either give up on life, develop secondary health issues, or re-injure themselves. Ruffian was another great filly who sustained life-ending injuries on the track; her owners attempted to save her, but when she same around from anesthesia after surgery she thrashed around so much she broke her legs again. Euthanasia was the humane thing to do, and if you think it was easy for her owners or trainer or jockey, ask yourself why Larry Jones or Gabriel Saez were too upset to interview.