It's clear Stern makes almost all of their money during the pre-order process of a new title release. After that, orders trickle in for the title and they need to quickly release a new one to get that chunk of money in again.
Example, Aerosmith. Right when BM66 is released and the checks have cleared, it's on to the next game and promoting that and collecting pre orders.
Since they have already made their money, what's the incentive to continue to improve the code? They will tell you anything to pump up the latest pin, then move onto the next release. BM66 is old news... time to get all that money in for Aerosmith now.