Because he was 17 at the time is the likely answer.
Which is the wrong answer.
2006:”On November 8, Malvo was sentenced to six consecutive life sentences without possibility of parole...As Malvo was 17 when he committed the crimes, he cannot face the death penalty, but still may be extradited to Alabama, Louisiana, and other states for prosecution...On June 16, 2006, Malvo told authorities that he and Muhammad were guilty of four additional shootings in 2002 (_in other states)”—Wiki
And why up for parole if no poss. of it?”On June 21, 2018, the United States Court of Appeals for the Fourth Circuit unanimously upheld a lower court’s decision that Malvo’s sentences of life without parole were unconstitutional. Judge Paul V. Niemeyer wrote in the decision that “Malvo was 17 years old when he committed the murders, and he now has the retroactive benefit of new constitutional rules that treat juveniles differently for sentencing.” (ibid.)