This is an important factor:
It's not about passage of time, or the life span of those immediately involved, but the impact on society.
Sure, you can have any amount of movies, plays, guided tours on Jack the Ripper - an industry in itself - but what is the impact of the brutal killings on Victorian society and, really, ours?
The mystery of the 'lost' victims of Brady/Hindley is solved ... these were the killers. But what was the impact? That a woman (cue the *gasp*) could've been involved in such brutality against children? When the Rose 'n' Fred West came out, the horror wasn't against the poor young women who'd died but the murder and death (and the strongly possible sexual abuse) of their own daughter.
Any amount of torture, abuse and disgusting horror is - and sadly will be - committed against children. The humane majority of us see them as a beautiful, future generation which should be protected and nurtured. Crimes against them are damaging not only to the person but also to society.
And we can mull over past criminal cases of the destruction of children but, we do so with the responsibilty to ask not only "Why did this happen?" but also "How can we prevent it?"