The pizza transaction was an artificial construct where both parties perfectly well understood that at this point in time there was no market for Bitcoin. They have colluded to create an artificial transaction using newborn currency in which they both believed (because anyone with a brain after reading the paper could see that it is and genius and elegant system that could work).

They have artificiality bootstrapped market for trading goods and services for Bitcoin by demonstratively performing the first public transaction of this kind. The rest is history.