Strangely enough, it all comes down to the Industrial Revolution and what it did to our modern diets. Thanks to our rapidly increasing production capabilities, in a period of a few hundred short years, we've gone from almost no one being able to afford sugar to having a diet best described as "Skittles." It's important to note that sugar, in and of itself, doesn't hurt your teeth. But the bacteria in our mouths has an even bigger sweet tooth than we do, and when we eat too much of the blessed white stuff (and we always eat too much of it), those little buggers dig into an all-you-can-eat sugar buffet, which gives them a crippling case of the acid-splatter-shits (all over your mouth). And that stuff is straight-up terrible for your teeth's enamel.
All of this would have sounded like witchcraft to our great-great-great-grandparents, to whom the concept of tooth decay was almost as foreign as that of television or not giving toddlers whiskey and tobacco for their birthdays. Researchers have theorized that we've simply altered our lifestyle far too fast for evolution to keep up with -- our bodies evolved to maintain the perfect balance of oral bacteria, but the Industrial Revolution hyped those little bastards up on a sugar rush from which they've yet to come down.
Food with flavor was a hell of a drug.