We take better care of our pets, don't we?
You see a news story about the impending starvation and removal of water in order to kill an animal and you'd have an enormous outcry. Most people are so callous and self-centered that looking upon the disabled makes them uncomfortable. To remove that discomfort, one, in their minds, has to remove the person causing it. Killing them and calling it euthanasia, mercy killing, "removal of life support", etc. sanitizes it for them.
Heck, how many times here on FR have we seen the uninformed posted type, "I wouldn't want to live that way." It's real easy to say kill the disabled when you yourself are not.