I think it has to do with them being a child and probably not being fully recovered from the incident. As a child Minora might not understand what they did was wrong or even hurtful, they might not know that that's something they have to apologise for [who knows]. And as for not being recovered, we have to remember that sexual harassment can leave really deep scars on the victims, no matter how "small" the incident seemed, and also we have to add that Minora most likely couldn't tell their family because of the circumstances or fear, so they've only got themselves to deal with this, it's clearly harder to heal like that. Maybe they are just not ready to fully deal with it and therefore be able to feel like they need to apologise. [this is my theory at least].
I thought Minora’s response to Kaname was a bit harsh (during the festival scene). Especially when you see the lack of apology from Minora to Kaname. I mean Minora basically outted him public, kicked dirt at him, and told him to die and get fucked. Especially since there were later events that were about why it is inappropriate to out someone, I thought it was a bit weird how Minora said that they didnt so anything wrong so they weren’t going to apologize. Is this because Minora is a child and did not see the fault on their part? I understand how terrible and insensitive Kaname’s words were, but Minora’s response was also incredibly hurtful.