Why are people trying to exonerate convicted murderers?
I don't have an opinion on the AM case, so no dog in the fight so to speak. But I think because of JM's close proximity to the area, it's hard for people to emotionally accept there being 2 such monsters in such close confines. It "feels" better for some to believe there's only one responsible for all the tragedy in the area.
And you'll always have mistakes made by human beings (LE) that necessitates things like The Innocence Project where people were convicted in similar situations with a body but later exonerated.
Basically the hope lives that it's one Boogey man.