Not quite sure. GW has been letting the Sister's stock reduce for awhile now. For example you can't even buy boxs of Seraphim any more. They only come in single blisters. I'm not 100% convinced this is a marketing ploy to push old metal stock.
I think this is simply they wanted to bring SoB in line with the other books before 6th ed comes out. Because the truth is then the community would point fingers at the sisters and how they were the next "dark eldar" of 40k. So by giving them a rules re-write, it gives them time to bring them out proper for 6th.
I remember a large rumer dump about how they wanted to bring to focus the struggle between the Warp and their followers, and the mortal races of the universe. What better way to do this then with a nice new shiny Chaos Legion and SoB book.