I think it's a feminist issue because it's about women. That's not a value judgment implying the plight of female immigrants is more important than the plight of male immigrants any more than I think male DV victims are less important than female ones. All are equal and all wrongs are equally wrong. But, as a feminist, I am concerned about maltreatment of women and therefore consider this a feminist issue.
When it comes to exploitation of male migrant workers - labourers/kitchen staff/cockle pickers, etc., I think there is more public awareness of how these people are exploited. I have always been appalled by the mistreatment of immigrants, and was horrified about the Morecombe Bay tragedy, but I'd never stopped to consider female domestic workers TBH, and I think that's probably true for a lot of people. There was a post on here about a year go that opened my eyes to it. 
There is still a huge amount of ignorance about how many female immigrants are badly treated outside the sphere of prostitution. I think many people still labour under the misconception that immigrant domestic worker is basically a cushy au pair number. In a country where many still feel it's acceptable to accuse SAHMs of living off their partners, it's easy to see how people can whitewash the plight of these women thinking that it's perfectly acceptable to pay them £50 per week in return for being on call 24/7 because, after all, they have a bed and food provided for them.
Obviously no one on this thread would think that, but I'm pretty sure a significant proportion of the population does - not because they are racist or xenophobes, but simply because they haven't thought about it at all or seen the evidence. Getting that information out there is what, for me, makes this a feminist issue.