Because the problem is that the Constitution doesn't prohibit it.
Even worse, it's likely that the majority opinion of the Court has staked their claim on foreign law and jurisprudence.
See the majority opinion - they did indeed mention the weight of international opinion against executing juveniles (who are often the most dangerous and completely immoral killers of all.) As I posted on another thread, term limits and re-election requirements are the only real solution to this frightening problem. Those who live in states with Republican Senators (I live in California. . .) need to REALLY put the heat on. With all the revolutionary stuff going on, now's the time!!