But isn't "bugger" also offensive to many, albeit in a different way?

The problem is that language is a slippery thing which changes with time and place. it may be true that the reason national nicknames were used more casually in the past was because the people to whom they applied were looked down upon, but it may also be that they weren't considered offensive at the time. And perhaps people then were less inclined to take offence than today.

Hugill excuses keeping the word "nigger" on the grounds that it was universally used by both black and white sailors without derogatory overtones. In a multi-racial, multi-national industry it was a man's ability as a sailor and not his skin colour which mattered. Of course the black sailors probably weren't in a position to object.

I had a Welsh friend at university who was universally known as "Taffy", I don't think I ever know his real name. It was neither intended nor seen as anything other than an affectionate nickname.

When I was growing up it would have been considered very offensive to call someone "black", the accepted terms were "negro" or "coloured". Now "black" is the accepted term and the others are considered offensive.