Well, if the guy was a member of some hate group, they might have offered him a deal to testify against others. Or maybe they just wanted to save money on all the costly death penalty appeals that go on forever.
It came out in the news today, that in Mississippi law, for a homicide to get the death penalty, there must be an underlying crime, not just the homicide itself.
In this case, they first believe that the killers took his wallet, but that turned out to not be the case. Had they done so, or committed some other crime beyond murder, they would have asked for the death penalty.
While it sounds foolish on the surface, this makes sense, because homicides are typically either emotion based, which are most commonly domestic disputes, that the public believes does not rise to the level of the death penalty; or crime based, which the public believes does reach that level.