Quoted from JodyG:Parts are ordered in batches, and the machines are run in batches. Those batches of parts have a lead time, because they have to be produced by the third party. Thanks to the cancer that is known as lean manufacturing (also known as JIT-Just in Time), these parts are not just sitting around and waiting to be shipped. The problem is that manufacturers, not specifically pinball related, have gone from "lean" to "anorexic" as a way of life. So when something like a Pinball machine sells above expectations, there is no extra slack stock to fill in while new stock is being made.
It blows, but warehousing is typically one of manufacturing’s biggest costs. Distributors tend to further throw a wrench in things by relying on manufacturing to have a surplus of inventory. The whole thing is a cycle.