Because there's too many guys like you who want to grope every women they see.
1) if you're just friends, then it's completely out of order. You can't just invade someone's space. If they're letting you hug them, it should be a friendly hug, which it isn't if you touch them in a sexual area.2) even if it's your girlfriend, she's probably not looking for something sexual if you're just giving her a hug. This depends on the girl, some won't mind, but it can feel like you only want her for sex, because you're bringing sexuality into what should be a normal non-sexual thing.
because it's sexual and therefore inappropriate if you're not going out. because when you see regular people hugging on the street who aren't couples, they don't touch each others' asses. therefore, because it's socially unacceptable.
cause the butt is considered a more sexual area maybe and sometimes girls don't like it when you touch them there
Why is it OK to touch the back, but not the butt, when you hug a girl?
I want to touch the butt.