I think part of it was just too much food! They're so filling.

That's a good point. During the strikes the Martin Brothers footed the bill and their goal was to make a filling, inexpensive sandwich. I went to look but I'm not sure what the main filling was back then - I figured roast beef would be cheapest. It sounds like if you wanted to buy a po'boy back in the 1920s it would cost you 15 cents.