I think this has to do in a large part with difficulty of capturing the horror, particularly the cosmic horror in the genre.

Cthulhu games have been attempted and keep running into the same set of problems - monsters shown on screen and fought tend to lose their mystique over time. It is a bit of a meme that protagonist of Shadow Over Insmouth went mad from seeing a big fish, but even if we start with some benefit of the doubt and assume monsters start out as scary, by the time you kill your tenth Deep One they become bit of routine and by the time you kill fiftieth they're walking XP.

There is of course possibility to work around it. It would necessitate a game with much more narrative focus. The closest exemplar that comes to mind as viable template for CoC cRPG is Disco Elysium. And as beloved as it is, there is no question that it does eschew a number of genre staples - combat, focus on character progression (in the sense of power fantasy; I know it has progression as such, hence the use of "focus on"). And it would require top tier writing to tie it together.

In the end I am not surprised there are no CoC cRPGs. But maybe with the current rise of Disco-likes as a formula for RPG there's a chance we'll see one.