While I don’t see that many horror themed games on the front page or the new releases my hypothesis is this: T’is the season. ;) I think October just makes people more interested in season-themed stuff. You could enjoy fake witches hats and pumpkin decoration all year round, but there’s just more interest in those kinds of games. And Halloween isn’t a surprise event or something, so developers can/could polish up their Halloween-themed game well in advance but wait with the release for a few weeks until there’s more seasonally motivated interest. Plus: Right now kids couldn’t go Trick-or-Treating, so there’s probably additional interest.
That being said, actual horror games seem to be rather hard to make. Most self-proclaimed horror games I have seen in recent years could be more fittingly labeled as shocker games. A jump-scare puts players on edge - but they aren’t horrified by the content.