Happy New Year,
You're the first this year... Every year someone asks this very same question. There are plenty of reasons why too... First is Liability, I would guess. Not all PPC's are a PPC...., Huh? Pindell Palmisano Cartridge is PPC, but many BR shooters have changed their reemer design and still called it a PPC, slight changes here, slight changes there. Still a PPC right?, maybe not! No ammo or brass manufacturer in the world is going to make that bet. You could have thick neck brass in a tight neck chamber, your brass could be 1.525 long, not 1.515 or 1.500 like some chambers. call JGS, Pacific, Clymer, etc ask them how many different PPC reemers they have, then figure out how you could make Brass that would them fit all of those chambers and be safe in all of them.
Second reason is there really aren't that many of us shooting PPC's. The .220 Russian wasn't made for us, it was made for the Russians. Pindell and Palmisano just figured it was the perfect starting point for their new Benchrest Cartridge. There are now plenty of other wildcats built on that cartridge that it remains the perfect base for all of them, why re-tool to build PPC for a few guys when you can build .220 Russian and build close enough for all of them, and have the liabilities for none of the wildcat variations?
How many Harleys dont get new handlebars, or new bags, or new wheels, or new pipes, or a new tank? Afterwards they're all Harleys aren't they? These are all PPC's too.