Instead of simply ending the movie on the finger snap, they showed everything that resulted from the snap and closed the movie out with Thanos’ bittersweet victory.
Per Christopher Markus’ remarks about the film’s ending to THR, the creative team didn’t want to simply end the movie without finishing the story they were telling.
Instead of hitting the “pause button” until next year, they ended this story to make room for the next one.
So, while Avengers: Infinity War does leave audiences with some critical questions, it also offers up enough answers to stand as a single story, rather than ending on a cliffhanger of who died and who didn’t.