I've been known to put just about anything on my pizza. Sweet, sour, savory, drugs. I'll try everything at least once. Most of the time the combinations are a horrible idea. Even something as simple as anchovies actually sucks, despite being well known. I don't care how soft the bones are. I don't want to eat bones. With that being said, I've definitely done sweet pizzas before. I even have a great recipe for a dessert pizza. The thing is, that pizza uses cream cheese, not real cheese.
The textures of chocolate and melted cheese seem way too similar yet different to be pleasant together. Believe me, I've taken alternate bites of a cheese pizza and lava cake, but there's some chew time in between those bites. It's different when you get a mouth-load of chocolate and gooey cheese.