Quoted from jeffspinballpalace:

Stern can churn out 15k pins per year and does, allowing them to pay their employees and make a profit. Problem is their customers don’t all want the same title, so they must build a mix of titles to create enough total demand. The annual demand for any title varies from 500 to a few thousand units so they need to run current titles plus new ones.
Demand for a title is influenced by price, yet Stern doesn’t primarily use pricing to alter demand. Instead their strategy is to release a variety of titles and keeps their pricing level relatively high to higher.