Serial Killers are a different beast. Some of them don't get put in general population with everyone else, some get their own perks in exchange for information or cooperation with authorities. I've read and understand that some of these guys actually get a kick out of reliving past murders and few even continue to torment victims families from behind prison cells via letters, interviews, etc. I've heard cases where certain ones got married, fathered children and even some who "found God" and became mentors to other prisoners.
Serial Killers themselves will tell you if they ever broke free they would most likely do it again.... We know it's damn near impossible for that to happen but really, why take that chance and why give them the pleasure of reliving those moments out in their heads. On tax payer dollars no less. There are people who don't have a bed to sleep on or food to eat but those scum do.
Theres a few reasons.