The advancing Culture Of Death. Its easier to kill someone who's not young, healthy, or beautiful. They're a financial burden and when they're in a coma, they're not sentient. So a rationale has been created to kill people that don't look like society's image of a person. The disabled fall into that category. People think Terri Schiavo's case was unique but I prefer to think of it as a bellweather on society's attitudes to the least among us.
(Denny Crane: "Sometimes you can only look for answers from God and failing that... and Fox News".) It's not society as a whole, it's the insurance industry. I've always thought the whole issue is really about money. If the family hadn't been paying out of pocket the insurance company would have put her down a long time ago.