Board Thread:Movie Discussions/@comment-27637441-20160128232821/@comment-25285745-20160203015743

But if you want my hypothetical in-universe explanation, the only person who probably knew about the Guardian Monsters and how to awaken them was the prophet Isayama. He was alive then, but he's the only one that would have actually bothered to awaken them. He might not have even been aware of the legend at the time. Following Godzilla's apparent death, he might have sensed that Godzilla would return one day, so he spent the next fifty years trying to find a way to stop them, and eventually stumbled upon the legend of the Guardian Monsters, Then again, it's implied Isayama is a ghost anyway, so who knows.

But aside from that, the first Godzilla's attack didn't last long enough to merit awakening the Guardian Monsters. He appeared in Tokyo one night, then a little bit later he returned and annihilated the city before being apparently killed by the Oxygen Destroyer shortly after. By the time of GMK, Isayama had already been working to awaken the Guardian Monsters for some time and was aware that Godzilla would return soon. Basically, the 1954 attack happened and ended so quickly that there was no time to awaken the Guardian Monsters, but by 2002 the prophet Isayama was working to awaken them as a preventative measure before Godzilla surfaced again.