That's because being called a white so-and-so is no big deal.
I mean, who really cares? Even Andrew Johns called himself White Trash which is something plenty of white people throw around - I doubt many people find it offensive.
But plenty of black people - especially kids - are ashamed of it, and throwing 'black merkin' around doesn't help them.
Of course, a lot of left-wing hand wringers (most of them white) are trying to make white people ashamed of their race, and they're enthusiastically helped along by racist idiots like Anthony Mundine.
In a perfect world people would be proud or ashamed based only on their own achievements, and for the people who feel safe, secure and prosperous this is generally the case. That's why most white people don't care about racial slurs directed against them.
But as people become more scared, uncertain and insecure (for example, poor people who live in hostile environments) they seek refuge in the group - generally people who look like them and talk like them - and that usually means race.
So since aboriginals are generally subject to the constant threats of violence and poverty and hopelessness, they are more likely to identify strongly with their ethnic group, making racist slurs like 'black merkin' that much more hurtful.
In the same way, it's usually the poorest white people who go on about white pride and sh*t like that - the uncertainty in their lives makes them identify more strongly with their race, and so they're more sensitive to things like being called a 'Captain Cook merkin' and so on.