Beats the heck out of me.
I suppose the 460 was just overshadowed by the 500 (undeniably can throw a larger bullet, but the 460, being able to shoot the less expensive and more available 45 Colt and 454 Casull seems to me like a natural, and gives up almost nothing in power until you get to the really heavy bullets).
The 480 Ruger was just overshadowed by the 475 Linebaugh and had bad press on account of 1) sticky extraction 2) rim interference, both of which would have been cured by the 5-shot cylinder (which I would jump on with both feet if there was one in 7.5" barrel). Availability of brass and bullets makes it a handloader's gun. If you go that way, the Linebaugh is more flexible. So, the 480 was destined to be a gun with only a small fan base.
A lot of that fan base was sopped up by the 500 S&W, 475 and 500 Linebaugh (and similarly powerful single actions) and the 454 Casull.
Too bad, the 480 is an excellent cartridge in a terrific gun, if only it had that 5-shot cylinder.
The above is almost 100% speculation and opinion.
Lost Sheep