Anchovies
Let's give fish particles their due, shall we? They're not really very good tasting, but they serve the dual purposes of being a funny topping and being the topping nobody orders. Say you're telling a joke, and the guy in the joke orders a pizza. Just put anchovies on the pizza and it doubles the comedy, if it's a pretty pathetic joke in the first place.

As for the second point, the fact is that there has to be one topping that nobody orders, and if there wasn't anchovies, the entire Canadian Bacon market would vanish and Canada itself would collapse into ruin, forcing us to send them billions of dollars in aid just to keep the "For Better or For Worse" cartoons coming.