"Why?"
For me, the comparison of headspace designs have to have context. Different horses for different courses, and exceptions to every rule, and all that... Rim-spacing cartridges don't make much sense in a bolt gun, shouldered rounds have challenges in a revolver, and mouth spacers have their challenges in big bore, semiauto rifles. Any of it can be made to work, but some take more work than others.
For a semiauto with a relatively big recoiling cartridge, I favor a shoulder when one can be had - contact (x), consistency, and forgiveness, and the opportunity to decouple my crimp from my headspacing. Case mouths move a lot more than shoulders throughout their life, and take a lot more touches than shoulders. Shoulders blow forward and are sized back, that's it. Case mouths stretch out, shrink back, are sized in, compress forward, are trimmed back, cut square, deburred and chamfered to a bevel, squeezed into a crimp... If I want to be sure my BCG and spring don't shove my bullet deeper into my case; in a shouldered round, I don't have to play games, just crimp and be merry.
For the context of the two, there's not so much difference. The 450 Bush isn't a straightwall, it's tapered, but a guy has to size the base/body appropriately to match their chamber, else the brunt of the feeding is caught by the case mouth. The shoulder on the 458soc isn't large, and it equally has to be positioned correctly, albeit with a bit more forgiveness. There's no balancing act with shouldered rounds, however, so there's no real consideration for headspacing vs. neck tension to fight bullet set back or crimp jump, and no messing with collet crimp dies to give sufficient tension AND mouth lip.
Mouth-spacing rounds run fine when loaded properly, but I prefer to have the shoulder.