The products also have/had a lifetime guarantee - providing you could find a rep and they still made the product... Got a jug replaced after 40 years of hard service.

It's also why the party model failed them - MLM for a product that never broke or wore out.

Newer tupperware was microwave safe.

Reps got a cut of party sales and if they made enough each month the could get other benefits as well (a company car, for example).