They were cursed with a terrible spell and became the "cursed people". They were cursed by having their species to die off slowly and were forced to keep their identities hidden.
The 'Cursed People' are the people who were cursed with a terrible spell because they betrayed Zahard and 10 Great Families after their great journey ended.[1] It is unknown what kind of spell was used, but the effects are cruel.
According to Hwang, one of the three Big Breeders of the Hidden Floor, also known as Big Breeder "Yellow", the spell effects are as follows:[1]
1.) They were born with a male and female side in a single body and either lost their reproductive ability;
2.) Had their original power sealed off;
3.) Or the chances of them having a stillbirth increased to the extreme even if they got pregnant, so their numbers were on the decrease.
Ha Yura's father had the the third curse. Yura and her twin brother also inherited the curse, but Yura was able to be freed from the curse thanks to the blessing of Ha Family.
Aka Williams (suspected, predictions), the first curse.
Trivia & Notes
The nature of the orignal betrayal is unknown, but fans believe it has something to do with them choosing to fight alongside Arlen Grace and V's resistance against Zahard, an event known as 'The Great War Era'.
Moontari note that Ha Yura and Aka Williams have similar vibe.[2] This might relate to them having bloodline from cursed people.