Since the 74HC47 apparently exists, and therefore renders the "HC" version entirely obsolete. But does not appear on eBay.
This would appear to be an "inertia" thing - minimal use in new designs (because you do not drive single 7-segment digits), so little demand, therefore price on HC version remains relatively high compared to an antique fab churning out LS. Probably a significant demand for service replacements with people not realising that not only could they, but should replace automatically with the "HC".