In a normal case, granting guardianship to the spouse would be the reasonable and correct thing to do. Among other things, the spouse would normally be expected to live longer than the parents and has also typically taken a vow to love, honor, cherish, and care for, in sickness and in health.
The problem in this case is that even if 99% of the time a willing spouse would be the best guardian for a disabled person, occasionally someone will end up disabled with a spouse like Schiavo. Usually this isn't too much of a problem, because usually someone won't end up disabled with a husband like Schiavo and a judge like Greer. Unfortunately, that particular combination looks like it can be deadly.