From the NY Times, it appears that the Missouri SC ruled it unconstitutional on the cruelity language in the Constitution. The SCOTUS upheld that opinion today.
http://www.nytimes.com/2005/03/01/politics/01cnd-scot.html?hp&ex=1109739600&en=1042460ab48186e2&ei=5094&partner=homepage
From the article:
In August 2003, the Missouri Supreme Court overturned the death sentence, deciding by 4 to 3 that subjecting a juvenile killer to execution was unconstitutionally "cruel and unusual punishment" and resentencing Christopher Simmons to life in prison without parole.
The state of Missouri appealed (Donald P. Roper is the superintendent of the prison where Mr. Simmons is lodged), asserting that the state high court lacked the authority to reject the United States Supreme Court's 1989 finding that capital punishment for 16- and 17-year-olds was constitutional.
When the case was heard in October, lawyers for the defendant argued that new medical and psychological understanding of teenagers' immaturity validated the Missouri court's ruling. The United States Supreme Court upheld the state court today in overturning its own 1989 opinion.