Blood circulation is a big deal in a horse's leg- they don't have much blood supply. Also sheer weight on not only injured limbs but extra on good limbs when severe injury. Horses are hard to immobilize, and if immobilized then other life threatening issues can occur. It is a tightwire act to keep them from moving enough to reinjure yet moving enough to keep their system working properly. Horses are a lot more delicate than they appear to be. They are not always cooperative patients either, some will relax and accept treatment & rehab- others will fight against it.
Even with all the strides made in equine treatment, trying to save a horse with severe leg injuries is an uphill battle, throughout the whole process.
As an owner of a blind, diabetic Lab who is a member of my family this is sad.