Re: Why these techniques?
I don't think it is so much the techniques, or rather, the form of the techniques themselves that are the discriminating factor.
Of the 6 pins/controls, all of them, except yonkyo, is common to many martial arts. Likewise, the general form of kote gaeshi, shiho nage, koshi nage, juji nage, irimi nage, and kaiten nage are common to many martial arts, albeit, performed quite differently.
kokyu nage is the odd one out, although there are similarities in the internal CMAs.
Notwithstanding the fact that the technical basis *may* have been initially derived from its DTR parentage, I believe the difference lies in the way the techniques have been adapted (evolved?) in aikido training.
Personally, I believe that the techniques, as they are (meant to be?) practiced in aikido, is kiko (qigong training). This belief has been corroborated by several senior practitioners (not all were necessarily aikido practitioners) I have spoken to and exchanged ideas with, but it remains largely my personal beliefs and YMMV.