I think it's mostly the weakness of the pre-behavioral economics. The status quo was economics that, almost on principle, ignored the way actual humans behave. Any amount of consideration of how actual humans behave, however limited or flawed, will look like an improvement. It's much like the way behaviorism in psychology, which seems lacking nowadays, actually looked good compared to the Freudian school of psychology it was displacing.
The Wealth of Nations was not the status quo, though. The neoclassical model of economics was much more committed to a model in which the individual maximizes some utility function. The neoclassical model certainly built on Adam Smith, but by no means the same as, and one of the differences was that neoclassical economics tried to find ways to ignore or abstract away anything to do with human psychology.
