Back home my grandfather has attempted to raise a couple deer he has found that were injured or too young to take care of their self. He tended the injury of the first one and kept it fed. It died anyway. He took care of the young one like a father, but it did not last either. Now, he wasn't a vet but he did tend a large variety of animals on the property: Russian boar, 20+ head of cattle, chickens, exotic birds from Macaws to African greys, your typical ranch dogs, geese, peacocks, goats, and so on. His explanation was that they just don't do well in captivity. Maybe a fully grown deer that was already healthy (we see how well that worked out) would do well, but a young/sick/injured one probably isn't going to make it.