Why was the final landing scene selected for the Sully movie poster?
It clearly show us that the pilot landed the plane on the water somehow and thus reveals a significant part of the film's story.
Because the film is a dramatization of real events, most people (in the U.S., anyway) already know that Sully was successful in landing the plane. The film is designed to show people what they may not know, all of the drama before and after.
Showing the plane's successful landing isn't a spoiler because it's a matter of public record. From Wikipedia:
Sully (also known as Sully: Miracle on the Hudson) is a 2016 American biographical drama film directed by Clint Eastwood and written by Todd Komarnicki, based on the autobiography Highest Duty by Chesley Sullenberger and Jeffrey Zaslow. The film stars Tom Hanks as Sullenberger, with Aaron Eckhart, Laura Linney, Anna Gunn, Autumn Reeser, Holt McCallany, Jamey Sheridan, and Jerry Ferrara in supporting roles. The film follows Sullenberger's January 2009 emergency landing of US Airways Flight 1549 on the Hudson River, in which all 155 passengers and crew survived with only minor injuries, and the subsequent publicity and investigation.