The main reason for the move is Transformers 6 had the same release date (and the Jurassic World sequel being scheduled to be released two weeks later) but some may wonder why move it so far into the future. Other thing is 2018 is a pretty full year release wise, there are a number of major movie releases. 2019 is far less crowded and March will put it far enough away from the two biggest releases Avengers Infinity War Part 2 and Justice League Part 2 (May and June releases).
There is possibly other reasons that have not been said to explain the length of the shift. Pacific Rim was originally scheduled for March 2017, a little over a year before Godzilla 2 original release, it was moved to August 2017 when Kong: Skull Island was moved to March 2017. Pacific Rim has since been pulled from the calendar but with a recent announcement of director Steven S. DeKnight being added to it the possibility of Pacific Rim being added back to the calendar is likely it would not be placed in the vacated spot from Godzilla 2 so it may get a later 2018 date.
Another possibility is Monsterpocalypse, Warner Bros got the movie rights to this kaiju board game and it would need to be added as well to the calendar of releases. There could be other simpler reasons like a normal delay (because of director or actors involved) or just that they want the two Godzilla movies closer together. In any case fans just need to be patient and hope for a great film(s).