They just need the same attention as the other armies, even for just one release. A lot of why they aren't popular is just that. They'd start an army if the line had something to offer, but when everyone is used to plastic troop kits, starting an army of solid hunks of metal for troops doesn't seem so appealing.
Right now the Sisters don't compete with other armies, and I'm not even talking about the rules. The model range is easily the most out of date in all of 40k, and as many have pointed out, there are many who would say "Hey, I want to start a new army", consider the Sisters, then throw them out just because of the all-metal line and pick something else. Why pick a more expensive and drastically less customizable army when you can do one of the other dozen or so?
Just one pack of plastic Sisters would make that go away, and it will take that to actually see if the army can stand on its own merits. As it is the only Sisters players are the ones who like the rather limited fluff enough to accept starting a 3rd Edition army with 2nd Edition models at 5th Edition prices. A bit dramatic but there you are. At least one of the three may be fixed now.
Speaking of fluff and books though, Black Library is at least coming out with a new book and an audio drama of all things for the Sisters later this year. Then there was Blood of Martyrs for Dark Heresy this past Winter, so at least they've got something here and there.