Board Thread:Movie Discussions/@comment-27637441-20160128232821/@comment-25285745-20160201210227

The simple answer is because GMK is a direct sequel to the original Godzilla, which never mentioned the Guardian Monsters. It would have been pointless to try and retcon the events of the original film to incorporate the Guardian Monsters somehow. It's easier to just keep the original film's story as it is and incorporate the Guardian Monsters into the new film's plot.