In medicine, it's a regular trick to find some new patentable formulation or tweak to switch to manufacturing shortly before the original patents expire, so that you continue to have a patent-protected product.

It would not surprise me if something similar was happening here: HP's current cartridges have at least one piece that is under active patent protection and they don't manufacture any designs that have no patent protection. I don't know enough to state that's the case, but it wouldn't surprise me.