Because these men tend to be clustered in poorer areas, their victims are mainly working-class women and girls. As was pretty much admitted (finally) in Rotherham etc, those girls just weren't considered important. I know it's been said that they weren't believed when they reported their ordeals, but I strongly suspect the callousness and cowardice goes deeper. I suspect they were ignored whether they were believed or not
And because they were thought to be no better than they ought to be. There were comments to this effect on the Begum thread, when people were saying they were quite happy to go with the men because of what they got in return.