You need to read a little more history. In the past (and this is a very basic explanation of this), with black people referred to a "coloured", they were routinely denied entry to areas open to whites for the reason purely of their race, the colour of their skin. "You're coloured, you cant' go there" "You can't use that, you're coloured". If you throw in another insult, for good measure, you just say "black ****". It has so many insults thrown in to one, the assumption that they are sub human, a second class of citizen, an inherent sense of attacking them personally but also belittling, focusing on the differences and the historical connotations they have - and while things have certainly improved, there is certainly still existing prejudice.
The "Croydon ****" or "Scourse *******" insults just don't have those connotations behind them.
To quote Mike, "And some organisations see certain things as worse than others, usually in an attempt to resolve previous injustices." - agreed, but also they are highlighted to such a degree because it is seen as important to not retreat back to acceptable racism even in a "mild" form, it is seen as more beneficial to not return to those days entirely precisely by taking such a harsh stance on it.