In the 20th century, folding bikes didn’t have much of a following. That “fold” usually meant that you’re riding a flimsy bike that may or may not be able to withstand a hard bump on the road.
At the same time, they weren’t even all that foldable to begin with. But today the best folding bikes are made with modern materials and with innovative designs, and they have become stronger and more dependable.
Consequently, they’ve become more popular as well.