Following the announcement that Star Wars: Episode VIII was delaying their own release date into December 2017, Avatar 2 was pulled from the release schedule entirely with no new date.
When Avatar 2 was first announced, it was supposed to be released in December of 2014.
Eventually, the movie was delayed into 2016, and then 2017.
However, due to the epic script that James Cameron was writing, that eventually became three movies rather than the previously planned two, it simply took much longer than anybody expected.