They virtually eliminate distribution costs by going direct to the consumer. Organizations that try to fight change almost always lose and are swept into the dustbin. I remember when everyone who took advanced math in high school had to buy a slide rule. The Post Company made a lot of money selling slide rules, although it didn't manufacture them. I have a Post slide rule at home somewhere.
Pocket calculators are faster and more accurate.
They can be programmed. Today slide rules are museum pieces. Post became Teledyne Post in 1970. In the mid 70s, the company stopped selling slide rules. Or consider companies that manufacturer watch springs and gears.