"Either the thousands of contemporary writers were all part of a big conspiracy, or else the case for the existence of Jesus is thereby seriously weakened."

Or they had never heard of a minor, local, cult that did not make ground until a few decades later. After people had been travelling around spreading a tale in places where no-one would have read about it in any case.

A minor cult that may have taken advantage of written histories being less common that now to make up a few things.

Not that written histories prevent myths gaining ground.