Serious Eats - seriouseats.com

Tradition has alot to do with it, but there's a practical aspect, as well. The dairy component in a cappuccino or caffe' latte just sits there, in the stomach - you wouldn't put that on top of a meal, would you? By contrast, a cheese course (with salad or on its own) eats lighter than a dairy-filled coffee beverage does. The pieces are relatively small, and its more of the same savory elements that got you to this point in the meal in the first place. Gelato has other elements to it that make it perfect post-meal: its temperature, its sweetness, its mouthfeel, etc. Plus, re coffee beverages, there's already a superior, game-over drink to cap a meal, and that's a short or long espresso - those other drinks don't even enter the discussion.

These little food rules work - here's another: caffe' latte is most often consumed by the 16-and-under crowd after naps and school, as a merenda (snack). Adults drink either espresso or cappuccino in the morning, then espresso through the day. I guess they don't see the point of all that dairy once you've passed a certain age.