You haven't been paying attention then
And if you think that Sororitas are niche and unpopular, you are either living under a rock, or your local GW scene is really weird
As a matter of fact, I have been paying attention, and I am certainly not living under a rock or in possession of a weird GW scene, thank you very much.
Historically, Sisters have never been popular, as before this new 8th Edition one the last proper printed codex they had was back in Third Edition. During 4th-7th Edition all they were given was the odd White Dwarf codex or digital codex and no new models. No other army has been treated so badly for so long. If they had been more popular, why didn't GW give them more attention before now? Try explaining that!
I for one am really bitter about all the attention that Sisters are going to get this year, because GW showed no such love for Bretonnia and Tomb Kings, yet at the time they deleted them they must have been more popular than Sisters because they had newer books and newer models, with (slightly) more attention given to them during Fantasy's reign. If any army should have been executed for being unpopular, Sisters deserved to bite the bullet. And for anyone who says that Sisters are more original than Bretonnia or Tomb Kings, all Sisters are are female Space Marines with the mannerisms of nuns, accompanied by a load of deranged priests and mutated monsters. They have weirdness, yes, eccentricity, yes, originality, no.
True, GW are considering bringing back the Old World eventually, but if they don't give Bretonnia and Tomb Kings the same attention as this new Sisters release, I will certainly consider boycotting it.
That's mainly because the Xeno(alien) armies aren't on the same power level as Imperium.
Eldar (elves) and Tau are the only factions that are strong enough to face Space Marines right now. Necrons, Tyranids, orks are in a really bad spot, if GW took the time to actually invest in these factions you'd see more players going to them and buying more stock.
Only because GW are bloody biased towards Imperium vs Chaos crap, one of the many reasons why 8th Edition 40K is the worst one yet.
And as for this New Year's Open Day, what a load of bollocks. GW had already teased the Warcry stuff and most of the 40K stuff aeons ago, the only thing really new being that new Commander Shadowsun (and given that the more humanoid aliens are in the second tier of bias, they didn't really need it). The only thing new with AoS is a poxy campaign book which half of the playerbase won't give a damn about.
Sorry for the language, but I am just fed up with the hypocrisy and bias of GW at the moment, which I believe is even worse than that shown by the GW of the past.