I have a few things to add to the above posts for more detail.
With a fracture at the knee, the joint would have either ended up fusing/having to be surgically fused, making locomotion hard, or it would likely have been damaged to the point of being doomed to be catastrophically arthritic = severe, constant pain for the rest of her life. Unfortunately, an arthritic horse can't be retired to a life of lying around the house 20 hours a day -- they have to be on their feet, both mentally and physically (and because they don't fit on the couch).
Some fractures can be fixed surgically or allowed to heal on their own (hairline fractures), but they are usually relatively simple fractures that do not involve joints, or that involve joints that can be fused with little problem (the proximal interphalangeal joint has very little motion in horses and is sometimes fused on purpose to stop the pain from severe arthritis). The larger the horse (and warmbloods tend to be on the larger side), the more likely they are to have other complications (such as laminitis) and the more stress they put on the fracture site, which also increases the demands placed on the plates (which may not be strong enough for, say, a femoral or tib/fib fracture in a 1500 lb warmblood), etc. In the textbooks on Google Books, there are a lot of instances where they say that fractures can be repaired if the horse is less than 500 lbs while horses bigger than that have a much poorer prognosis.
You can only put a cast on the lower limb (distal to the carpus/tarsus), and cast sores are common and problematic.
As mentioned above, not all horses take kindly to stall rest. Some require drugs long-term just to make them safe to handle while they are confined, and even then they may pace the stall and rear up, which obviously is not conducive to healing. I have a friend whose horse was just in for a couple days and started literally trying to climb the walls, rearing up and hopping on his hind legs while pawing up the wall with his front hooves, whenever they were turning out the other horses.
