This upcoming movie is based on superheroes and comic books and there are so many other similar movies in that genre. For example, Batman vs. Superman is set for release this year and the movie media likes to build up the promotion and previews for a bigger audience at the release of the film.
The Suicide Squad movie has been delayed longer than necessary. Fans have been waiting for this movie since 2012, and it was one delay after another. Although dedicated fans are still excited about the movie, excitement dies down when a person has to wait so long to see a highly-anticipated film.
Unless a movie is delayed specifically to create anticipation, it is pretty difficult for something to be delayed longer than it should have been, since it was delayed for a reason. The timing of a release matters in terms of gross income, and a delay allows for inevitable errors, problems, and polishing.
There are many elements that go into making a movie, and it generally takes a lot of time to complete shooting the film. After shooting, a movie goes through many rounds of editing and finishing touches before it can then be viewed by the public. Suicide Squad may have taken a long time to go through this process, but it was necessary to ensure that the final movie is as perfect as possible.