Which is what worries me, all the titles given are of standalone horror novels, not the actual Penny Dreadful series like Varney the Vampire (although I have the compendium volume, it's massive, so maybe that's why).
It's name recognition. Varney the Vampire is not exactly a household name. (Sweeney Todd kind of is, though, which makes his absence more noticeable).
On the other hand, a TV series cannot subsist on three characters alone. It could be that Varney and his penny dreadful kin show up for standalone episodes, as recurring characters, or as the rest of the main cast. It depends how much the show commits to being a pastiche of Victorian horror (and though not penny dreadful, appearances of Irish horror icons like Melmoth the Wanderer and Silas Ruthyn would be nice, there's more to the tradition than Dracula, even if his abdunance of media appearances would say otherwise).