No, I think you're right (my wife thinks so too, if that means anything). First of all, I think it would then be directed toward a male audience and while there seems to be general belief that most men have domination fantasies, my own life experiences tell that really isn't so. I think as a male-view novel, it would have been a niche book at best.
And within that niche, I'm sure it would still do quite well. There are those within erotica that like those themes, like to explore them, read them. I think that bears out quite a bit, actually, in the browsing of the erotica epublishers catalogues as much as I've seen, at least.
Perhaps what has brought it all to head at this time, with this book is the being at the right place, right time. The author had a book (whatever the origins of it were) that was doing well within a niche. Someone, somewhere saw it, read it and said 'this puts a finger on an untapped pulse'.
With the added avalanche of a previously built up fanbase through the smaller epublisher, the momentum just carried it along. It got a big advance, then it got news mention, women who don't seek out erotica (probably didn't know it existed in this form and thought all erotica was porn...which equated to porn being for men and therefore not of much interest to them) took notice, bought it and word of mouth carried it to new heights.
Here was something for women. Something men enjoy in the privacy of their basement, now they have something that is just for them. And it touches that chord in them because the MC is a woman, just like them. Perhaps they remember that time when they were young, naive, unjaded...and they want to recapture that. Add to it the dark and forbidding and unknown of BDSM that is only whispered about behind hands and with harsh hisses....
The more I look at it from this angle, it's not hard to see why this book managed to completely take off like it did.
*shakes fist* how dare you make me thinky think on this?!? /jk