From what i've seen when i was testing the same thing, the game has a limited number of slots for secrets. i think it's like 40-60 or so by default.
It will sometimes select custom ones, but the default ones are more likely, because it generates like 50 of them.
I had some success by reducing the chance of civilisations worshiping the Death and decay spheres and increased the chance of them worshiping the sphere that my secret was tied to [SPHERE_ALIGNMENT:(Sphere):0-25600] 256 is the default, according to the wiki. I set mine to like 100, and it seemed to reduce the number of necromancers, and resulted in more of my custom secrets.
In addition, you can tie your secret to multiple spheres IIRC. which obviously means it will occur more.
Edit: Wait, did you mean entity curses? or secrets? because they work differently, but i think it's a similar problem. it generates 50 odd potential curses, and your 1 secret has a 1/50 chance lol. i'm not sure spheres influence curses.
You might be able to use advanced generation settings to set the number of secrets and curses quite a bit lower, that seemed to increase the number of custom stuff too. i think those settings only affects generated curses/secrets.
Not secrets, secrets are no problem, talking vampire curses.
Another way though is to make multiple identical copies of your curse (with different IDs) to increase the odds that it will be the one picked. If you want to totally replace vanilla generated curses with your own, you would have to set the amount of curses in worldgen to none.
On world generation settings I put 3 vampire types and 3 necromancer types, I have a lot of vampire curses, about 9 or more, i see my custom secret always in legends, never anything about vampires eating or drinking or being cursed by the gods ??