Supply and demand. Demand ramps up, production ramps up (or becomes available to outside oem), prices go down.
Stockpiles can go up as production needs go down.
Also, as a product gets older, pricing can be dropped as it becomes “mature”. As a devices heads to eol, its profit margin might be eroded so as to milk the last bit of money. might not be applicable here but is on other products.
Why make aftermarket parts when a vehicle is still under warranty.
After 3 years, you do see aftermarket being more available.
Not worth it for those 10% of repairs not covered under warranty.
Which is why the dealers were pouring a 12oz bottle of water from the break room into the tank after trucks with obvious CP4 failure came in. They didn't want to do the work at warranty rates, so they didn't, and blackballed the trucks from warranty. "Sorry bud, we found water in your tank. $18k based on "book time" and no more warranty."
Back to the OP, it comes down to supply. As common vehicles age, demand is created for common failure items. This incentivizes the SKPs and Dormans of the world to tool up and produce. In the case of your CP3 and injector, they're ubiquitous enough that many aftermarket rebuilders have set up shop with various quality of reman offerings.