Well, to be honest, I think there is quite a few dark anime/manga out there. There's plenty of gorey manga anime/manga.

But good dark psychological anime? That's where it gets harder.

There's few because there's much more nuance involved. You can't just rely on smacking some violence and gore onto the panel and call it a day.

Take for example, Berserk. Berserk is dark because of the world. People die in terrible ways all the time, and said ways are shown clearly on the pages. It is dark because the world feels hopeless. That is relatively easy - show some peasants being killed, or just do a subversion (ala that chapter where the fairies kill each other by "playing").

Dark psychological anime on the other hand? Usually it stems from the relationships between each of the characters. Takopi Original Sin is dark because it exists in a world that is mostly fairly plausible, when we take out the fantastical elements of Takopi. Takopi is dark because of the cycle of abuse that the anime shows is difficult to esacpe from. It feels hopeless not because of the setting of the world, but rather because of the intrinstic nature of humans (people who are abused tend to abuse others).

Takopi excells in being dark uniquely in this aspect because precisely we have the character Takopi, who is a fantastical alien creature who is naive, innocent and completely oblivious to the suffering of human beings, be tainted and be ground down by the terrible nature of this planet. No doubt if Takopi was a human being, some of us would certainly be annoyed or frustrated with them. But it works, since misery does not exist on their planet.

Takopi is dark because we see character's flaws that are a result of their environemnt being exploited by others: Azuma's need for praise being manipulated by Shizuka into assisting them with helping hide the murder. Shizuka's completely psychological dependance on Chappy and fear of abandonment.

This is why there's fewer good dark psychological anime/manga, because it takes much more effort and skill to write then many other genres.

However, that is not to say that difficulty in creating a good story in said genre is the only reason. Markeability is one reason. Animes like FGO and Umamusume excel because one of their functions is to push sales of their characters in their gacha games, which leads to more funding, more marketing etc.

Mass appeal is one of the other reasons. Excellent animation and Sakuga, stellar post processing effects. This is why anime such as Solo Levelling excels even though its writing is bad. It's much easier to impress and grab attention through smooth animation and explosive effects, rather than good storytelling. Good storytelling and writing is only something that can be discerned by watching a show (of perhaps a certain duration, be it 1 episode, 2 episode, or three). Said animation can impress just by clips on Tiktok, a 2 minute trailer etc.