“The most telling aspect of The Conjuring 2, the gonzo sequel to the 2013 horror smash, is that it's 133 minutes long. A running time like that is a rarity—The Exorcist, at 132 minutes, may be the strongest analogue—because the genre draws intensity from concision, and its dread-soaked mysteries are not so easily sustained over time.”
I haven't seen the The Conjuring 2 yet, so I can’t comment on the movie. But there is a telling line in the above review excerpt, whether you’ve seen the film or not.
Horror fiction is difficult to sustain over the long haul, for all but the most skilled practitioners of the craft. The best horror works typically fall in the range from the short story to the novella in terms of length.
Note that neither Poe nor Lovecraft ever wrote anything that could properly be called a novel. Even Lovecraft’s longer pieces (At the Mountains of Madness comes to mind) would be classified as novellas.