I have witnessed this in the south too. I am worried that the media coverage is meaning people assume this is a problem only with asian men in northern towns when in reality grooming is common across a wider society. However I think there are some unique factors.
The situation I have seen were very similar to those in this case, but the girls lived with their families not girls in foster care. The men targeted young girls over women because they wanted sex, and knew they wouldn't be able to treat many older women like that and get away with it. The schoolgirls were young and naive and easily tricked.
White girls were generally targeted over asian girls as they were out on the streets more, allowed more freedom and so were more available. Also they wouldn't dare do anything within their own community where people would talk, it would be discovered and they would be punished, probably vigilante style. The segregation between the white and the asian communities meant they were able to keep it quiet from both their own families and the families of the girls. Speaking another language that the girls didn't understand was also useful, the gangs could speak about them and trick them easily.
I think it is a sexually driven crime, first and foremost but thinking of the victims of 'lesse/other' made it easier to commit.