Because if only Sol and Luna families have kids, it’ll mean that they ONLY have Sol or Luna and that would mean no one is having kids with their spouses. It also drives up the chance of Lunas being sexually assaulted. I think this is the imperial way of trying to protect them but it kinda backfired because as long as you aren’t trying to marry a Luna, you can still use them. This was also Joseon era and it’s quite possible the emperor wanted to have less Sols to compete with. By having the imperial lineage be Sols and everyone else be mixed or normal humans, it makes it easier to maintain control.
Those are just possibilities though and I don’t think anyone truly knows why it’s now illegal.
Why can't Lunas and Sols get married? I didn't understand the reason