Snarf Zagyg
Notorious Liquefactionist
I am saying that the baseline assumptions of D&D make it a bad fit for horror because those baseline assumptions counter almost every aspect of horror. Not just the character abilities. The fact that there are concrete rules at all is anti-horror. Horror emerges from uncertainty and a lack of control. D&D's rules provide both to the players by design and intent.
You can certainly create tension but that isn't the same thing. You can even create certain kinds of fear -- of failure, of PC death, of potential consequences -- but these aren't horror either.
Hmmm.... maybe you need to dwell down on what you mean by "horror."
Horror is a large genre, and most people would think that everything from the cosmic horror of a short story like "The Colour Out of Space" to the jump scares you find in a slahser fic are all within the "horror" umbrella.