The new breakfast item didn't take off at first for several reasons one of which is that it was called: "Country Squares." After starting off slowly they decided to re-brand it with the name Pop-Tarts, and it took off.
Then they hit another snag when lawsuits began piling up in the 90's because the Pop-Tarts were catching fire in toasters. Syndicated columnist, Dave Barry, wrote an article about his toaster oven catching fire. As usual, it forced a packaging change where they added the warning: "Due to possible risk of fire, never leave your toasting appliance or microwave unattended." This reduced their liability. It's amazing that with the millions of Pop-Tarts sold every year, Kellogg's has only had three small recalls on the product.
Pop-Tarts aren't a global sensation because not all countries are innundated with microwaves and toaster ovens like we are, but people who live in Ukraine, love them; especially chocolate and strawberry.
For a grocery product to last over 50 years and perform so well that they have so much shelf space in grocery stores is an amazing accomplishment for any grocery manufactuer.