"Hammer ... went back to the original source material, the novels, rather than merely copying the Universal films approach."

With respect, grg, I don't find Hammer's films to contain any more of Shelley and Stoker that Uni's; I think they forged a "third way" of their own.
You're leaving out the first part of what I actually posted - "As for why Hammer did their horrors in period..." I never said they did strict adaptations; they changed plenty along the way (often for budgetary reasons). The question, though, seems to be why Hammer started off doing their horror films in period rather than modernizing them (as Universal did with their Dracula). My point is that once you start with the novel (rather than the play based on the novel), you're already in period.