First, the molding technology of the times had the kit manufacturers creating tooling that was 100 times stronger than needed for the limited kit runs they had envisioned. As Bob Paeth of Revell told me, they were toy manufacturers and were focused on the current year. They had no idea that this would go on 50 years! Today's tooling is aluminum and designed to last only as long as the manufacturer sees the product being needed.
Second, the lack of organization of companies back in the era helped us greatly! They would just take the obsolete tooling and toss it into the warehouse. No inventory system, no deprecation schedule per tool etc. Yes, from time to time they'd crawl through the warehouse and come up with retooling ideas for old tooling such as the "Modified Stocker" series, but indeed they never saw the need to reissue these cars in stock form again at that time! And the fact that the companies changed hands a few times and that entire inventory was dragged from one place to another! Amazing good luck for us today!
Today we'd never see this with manufacturing companies. Every piece would be bar coded, they'd know exactly the value left in it, where in the warehouse it was located, how much that storage was costing, and when to destroy the molds for scrap value.