That's because this ONE judge in ONE case subverted justice by his prejudicial rulings. That's "unusual" too (we hope!), but it desperately needed review. The higher courts do not, as a rule, overturn findings of fact by lower courts. That is a huge flaw in the system. It allowed one biased judge to put an innocent disabled woman to death. He simply chose the evidence he wanted to hear and excluded or ruled against everything else.
The higher courts had a primary duty to uphold constitutional law, not to provide shelter for a rogue judge on the lame principle that you always do provide such shelter. Moreover the act of Congress specifically allowed for your review de novo. You had no excuse for playing politics.
There are good reasons for, as a general rule, requiring that findings of fact established by lower courts stand. Were such policies not in place, someone with enough money could win almost any case against someone with less money simply through attrition (endlessly seek do-overs until the desired result is obtained). In addition, the Constitution generally requires that facts decided by jury be allowed to stand.
The biggest problem in this case is that Terri Schiavo was never represented in court by anyone who didn't want her dead. Terri's parents had lawyers which sought to protect Terri's interest, but since they were the parents' lawyers and not Terri's, they did not have all the powers necessary for effective advocacy on her behalf.
Not sure how the law should best be set up to deal with these situations. To be sure, the existing laws probably would have been adequate had they been enforced honestly (in particular, if Michael's conflicts of interest had been recognized as being sufficiently obvious to preclude him from acting on "Terri's behalf") but perhaps more explicit protections from rogue guardians need to be legislated.