"You are looking for our Captain? Well I have not seen a brother Captain for a long time, Brother Librarian is in charge now..." -Communication log with unknown Blood Angel Chapter successor.
The curious trend was first discovered during a massive sub sector campaign where no less half a dozen of Blood angels successor chapters took part. Segmentum command reported almost all of the Sons of Sanguine they have came into contact were void of Captains, sometimes a Reclusiarch was in charge but most often then not a Librarian is in command of their strike force, a Captain was no where to be seen.
This curious trend of abnormal number of Librarians in power of a particular Astartes genus has caught the attention of Ordo Hereticus, a report was made to the highest authority of the order and a subtle "enquire" came into action.
What they found out was a disturbing under trend going inside most Sons of Sanguine chapters. It seems some newly discovered psychic ability weldable only by Librarians of Blood Angels origin has propelled them into very powerful position within their chapters.
The emergence of psychic powers like Sword of Sanguinius, Shield of Sanguinius and Unleash rage has amplified their prowess in any battlefield to the point a Captain or Chaplain would seem redundant or even obsolete in most combat scenario. A Librarian in a Blood Angels successor chapter is now crucial to the success of most sizable operation by increasing the survivability of their gunships, denying hostile psychic powers, increasing the battle prowess of their battle brothers or cutting down monstrous creatures and walkers alike themselves. It is said within the chapters a battle not participated by a Captain is unfortunate, but a battle not participated by a Librarian is disastrous.
Slowly and surely the Captains of such chapters found themselves sidelined from the Chapter's core of power as they now serves no more than over glorified swords man in a battlefield filled with hostile psykers, monstrous creatures and column of tanks -- all which the Librarians are better equipped to deal with. To add insult to injury, it is said a Librarian cost considerably less resources to be fielded than a Captain.
It would seem the Librarius was gaining so much power it eroded the authority of their Captains to a point they longer have access to artificer armor nor relic blade like standard Astartes chapter do.
While this trend is in no way violating any Imperial decree, the Inquisition is keeping a close eye on the development. In the meantime, seeing a Space Marine Captain from the Blood Angels line in combat is as rare as a space marine getting killed by a grot in close combat.