I'd bet money that @Shipwreck nailed it. The flat snap rings are a commodity part and easy to source. Wire versions would need to be custom manufactured and probably end up more expensive for a weaker component.You and @Feldrian are correct in that there is not much lateral pressure on the circlip. The two snap rings simply prevent the inner axle from walking side to side. The flat snap ring prevents the axle moving toward the birfield and bottoming out into the bell. The wire circlip prevents the axle from shifting toward the diff. I suspect Toyota used the wire circlip to make it easier to remove. RCV may feel that a heavier, flat clip makes a stronger product. Or, maybe a flat machined clip is more readily available and therefore cheaper than custom forming a wire. Who knows.
You've got your axles together. Run em.
And since I'm already hijacking your thread @Broski, anyone remember the Martack? Marlin Crawler began putting a small tack weld on the differential splines of the axle and then omitting the wire circlip. This made swapping a birfield easier if you grenaded one on the trail. I still have my FJ40 axes set up this way.