I think in theory escape risk is a part of it, too, and people with extremely long sentences are presumed to have rather more motivation to escape.
Elizabeth seems the sort of person most capable of executing a plan whereby she pulls off a prison break through enlisting the aid of a bunch of other prisoners who're promised freedom themselves, but in her plan are actually there as decoys to be killed. It seems analogous to stuff she's already happily done. She is potentially a risk even to other prisoners if she carries on as she has done. I don't buy that she's not a risk to others.
I’m not talking about just Holmes here, but about prisons in general. It’s not supposed to be fun, but neither is it meant to support a collective revenge fantasy.