Pineapple pizza fans love the contrast between the sweet pineapple and the saltiness of the ham, bathed in rich melted cheese and tomato sauce. What's not to like about that? It's an instant classic. Sweet and salty food combinations are seen the world over because they're objectively good.
Fruits are a classic accompaniment to many savory dishes, bringing out a sweetness and fullness to many meats and starches. Chefs all over the world enjoy mixing fruits into savory dishes: Applesauce on latkes, pears with pork chops, duck à l'orange, raisins in coucous royale, orange-scented Afghan rice, sweet and sour pork, prosciutto wrapped in cantaloupe, and quince jelly and cheese. I could go on, but you get the idea. Sweetness and a little tang (say, sour cream with the applesauce on latkes) are the perfect accompaniment for a main course.
Haters say pineapple pizza is a culinary abomination. Sure, sweet and salty things go well together—but pineapple and pizza do not. If that's true, why stop there? Why not watermelon pizza, or cantaloupe pizza, or some other inhuman creation? You have to draw the line somewhere. Besides, it's not as simple as throwing any sweet and salty thing together.