If he's eating from the dumpsters why not eat from the fancy restaurant's dumpster instead ?

I can think of a few reasons. People tend to overorder pizza more than other stuff. Pizza is also more robust than other foods, so it survives being thrown away more than other things. It's also very satiating, comprising bread, dairy and meat.

Right, but I'd expect it to be the same with fancy pizzas. I just don't get the point to target cheap pizza place in particular if you're going to dumpster dive

It might just be that there are a lot more cheap pizza locations than fancy ones.

This fellow also stays with friends.