It may get a lot of credit now, but at the time of its debut in 1928, sliced bread received less-than-rave reviews. Baker and inventor Otto Frederick Rohwedder had spent 15 years perfecting his bread slicer (finally settling on one that wrapped the sliced bread to hold it together as opposed to the hat pins he'd tried earlier), but consumers weren't quick to convert. People found the sliced bread strange and senseless.
It wasn't until the advent of Wonder Bread, and the collective realization that sliced bread worked better in the toaster, that Rohwedder's invention really took off. By World War II, the military was using sliced bread to serve peanut butter & jelly sandwiches as part of soldiers' rations. Previously uncommon, the PB&J gained a loyal following among servicemen, who kept making the sandwich, sliced bread and all, after they came back to the home front.
This explanation originally appeared in the "25 Most Important Questions in the History of the Universe" issue of mental_floss magazine.
* * * * *