You could have it that this Chapter is the result of a philosophical disagreement in the early days within the Blood Angels on how to handle the Black Rage. Those senior Blood Angels who disagreed with the foundation of the Death Company and saw it as dishonourable to use them as Berserks could have been a vocal minority within the higher echelons of the Blood Angels, and when the 4th Founding began they were given the option to leave and apply their preferred methods of dealing with the Black Rage within their own Chapter, rather than continuing to cause disharmony within the Blood Angels.
The higher rate of attrition really isn't the case - the Death Company die fairly quickly on the battlefield, and their progenoids are reclaimed. If anything the progenoids of the Archangels who fall to the Black Rage will be more easily recovered (unless they refuse to use them for puritanical reasons, as they see the Black Rage as a taint).