It's been a really long time since I've read the novel but I guess it was because helios requested kael as sort of a favour.
Avocato is right that was one reason but I saw his personal reason as he just wanted to move on from the whole Diana and Helios drama so he can move forward with his and Hestia’s relationship. He didn’t want to deal with their problems anymore and if she is stripped off her title it will cause chaos within their political world kinda like the nobles fighting over who the next crown princess will be, which means it’ll be more work to deal with and less time for him and Hestia. I mean I don’t like it but I do get it. （￣へ￣）
can someone spoil me why he doesn't want the bitch to be stripped of her title? or im gonna hate him like fr