I'd imagine the most prohibitive 'Why Not' would be -- what's the market for a pen like that?
How many people 'do' Copperplate &c, how frequently, & how extensively? You just don't have the requisite demand. You couldn't even recover the engineering costs, much less any decent warranty.

Otoh, I could see a self-made fledge that would fit onto the more standard handles, which could hold the Section of a regular fountain pen, as most feeds will write pages after dipping. Maybe even with a cartridge fitted thereonto...and write 'til your wrist collapses!