My impression is in the C18th, to read 'a book that wasn't true' was considered a waste of time so Walpole may have hoped for greater public acceptance with 'a relic of the past', an early Medieval tale.
At this stime in England (as I've explained in another [post) Englishmen had great cultural interest in medieval events, epecially set in Italy.

Yet, there was a fear of ridicule presenting supernatural or 'marvellous' events except through authentic products from an unenlightened time i.e old documents, translations of early medieval stories.
No doubt, it was more interesting for C18th readers to believe they were reading an authentic story, something true rather than a fabricated tale.

Actually, I'm reminded of NA, ch.21 when Catherine finds a roll of paper in the black cabinent in her room and blieves it is some manuscript of antiquity. Her candle was accidently snuffed leaving her in darkness to ponder it's contents.
Next morning (ch.22) Catherine persuses over the sheets, only to find washing bills !

'Nothing could now be clearer than the absurdity of her recent fancies. To suppose that a manuscript of many generations back could have remained undiscovered in a room such as that, so modern, so habitable !-'

Catherine's reaction to the laundry bills may've been similar to how Walpole's readers felt on finding out the authenticity of 'Otranto' as a medieval tale was a modern hoax.

Though of course, in Catherine's case, it was in her mind ! ;)
She imposed flimsy gothic values onto real life.

Actually, befor I read your post, the idea of horror movies put me in mind of a hoax perpetuted by smallfilmakers in the US a few years ago. I've forgotten the name but it was done as a docudrama and shown in cinemas round the world about 3 students who disappeared while checking out a 300 year old tale about a witch. Their car was found on the side of the road and 2 years later their film dug up.
Creepy huh ? ;-) It was all quickly revealed as a hoax, even the witch tale, but not before it made masses of money at the box office.
I think I planned to see it, but lost interest when the hoax was revealed. I wonder if the reading public felt the same way after Otranto was reveled as a hoax.