Willow was able to heal herself and the soldiers because all the physical matter was still there.
Xanders eye was completely deformed by Caleb, the remains would have been taken out during surgery at the hospital. Beyond a certain point of eye injury, the organ itself is cut out because they is no chance of restoring it. There was nothing left for Willow to heal in Season 8, just an empty socket.
The same with Vampires, for some reason throughout all Vampire fiction when they have a problem with their eyes it does not repair when becoming a Vampire, hence some of them still needing glasses or staying blind, they can't regrow eyes either.
Vampires without eyes though can take fresh ones from Humans and place them in their own head, the new eyes fusing and working for a few centuries before needing replaced.
So, if he really wanted he could take one from someone else, place in it and Willows healing ability would make *that* one attach perfectly and work, but that would just be gross.