The switch of grain-based diets is suspected to be a big part of the cause.

It's important to remember that hunter gatherers had much more diverse diets than agriculturalists (and therefore also much better food stability). I would posit that any member of a primarily agricultural society had terrible teeth. But we generally see this less in non-agricultural diets

That being said, when you have Inuit people eating mostly meat with moss being a major plant source and Ryukyuan people turning an extremely poisonous plant (cycads) into everything from flour to alcohol, it's important to note that ANY generalization about human diets (and the health outcomes heavily affected by those diets) is hard to generalize