Unless I've read that wrong, that's not fluff, but game rules that can be interpreted differently - either there just flat-out exist no psychic sisters or no sister with psychic powers would be allowed to remain in the convent and possibly be serving an Inquisitor in their cadre while still being considered a sister and given access to the standard wargear of the sisterhood.
If you prefer your interpretation, go with it - 40k doesn't have a "real" canon anyways, so even though I think that line is fairly clear, it doesn't really mean anything. As far as I'm concerned, it makes sense, as psychic potential was described to be genetically inherited before, and it is reasonable that the Schola Progenium would test any of its students for genetic purity (signs of mutation) at the time they arrive at the facility.
A few tends of thousands of Soroitas in the galaxy wpould make them completely useless as an armed branch of the Ecclesiarchy. That's probably smaller thatn the police force in New York City.
While it's certainly possible that an extremely small organization would be given an inordinarily large amount of spotlight in 40k (just like you can't throw a stone in a warzon without having it bitten in two by a Catachan - wasn't that supposed to be a sparsely populated death world?), a few tens of thousands would be.... well, next to nothing. Certainly nothing that was notable in a setting where wars routinely engulf entire planets (IIRC, our planet alone has about 20 million people in various armed forces, meaning they'd outnumber the entire Adeptus Sororitas about 1000:1).
I don't see the problem. There are only 1.000.000 Space Marines either, yet no-one is complaining about the frequency of their appearances in the fluff. The Ultramarines are 1.000 men, yet they are mentioned far, far more often than any of the six Major Orders of the Sisterhood that are several times as large. Same for a whole lot of other Astartes Chapters.
Similarly, the Imperial Guard's Storm Trooper regiment was said to be only 10.000 men strong as well. It's not a question of being everywhere at once, it's a question of having the right amount of troops at the right spot at the right time.
And note how 99% of the time, you really only ever hear about the Major Orders, who together have been said to have a maximum headcount of 30k in the Codex fluff. The smaller Minor Orders are more spread out, but these receive next to no mention because they just don't get engulfed in those "routine wars" as often, the most famous exception being the convent of the Ermine Mantle at the Cadian Gate. Whenever you have the Sororitas show up in force - which really isn't all that often in the fluff - it is always one of the Major Orders.
The (relatively) small size of the Sisterhood is, to me, the perfect explanation for why they show up in the fluff comparatively seldom, compared to the other armies. The first and foremost line of defence for the Ecclesiarchy is and has always been its warrior priesthood and the Frateris Militia. The Sororitas are a symbol of the Church Militant's might and a reminder of its power, but they are not its primary power itself. That would be the people. The worshippers.
In the words of the great Sebastian Thor: "A single man with faith can triumph over a legion of the faithless. Untold billions of the faithful can never be opposed."
This also ties in to the Decree Passive. If you have millions of Battle Sisters, with each Order armed with (in Codex fluff) "arms and armour the equal of any Space Marine Chapter", and even being deployed to purge rogue Astartes, I could see a lot of people becoming nervous about the Church's martial might. The low numbers of the Sisterhood are also a sign of its extreme membership requirements (the 2E 'dex lets you follow up on the very, very slow growth of this organisation which took several millennia to simply triple its size after Vandire). Plus, In my 40k, I also like my power armour to be a rare sight...
To be honest, I think that some people just don't like that, in GW's vision of the 41st millennium, the Space Marines are not the only rare and elite force of the IoM. If there are fans who don't like that (which is somewhat understandable, since their representation changes heavily depending on where you look, so a lot of people will have "grown up" with a different idea of them), there's no need to go this route. I just thought I'd point out how the original background describes it, as to at least raise awareness that there are several different options here.
Edited by Lynata, 11 August 2013 - 01:31 PM.