Posters have to be self-contained, in two senses of the word. This is a major difference between posters and slides.
First, you only have a single canvas for a poster. Everything has to fit within that space. In contrast, it’s rare to have only one slide. Slides are gregarious things, preferring to appear in swarms. With digital slides now the norm, you effectively have infinite space (even though you see only one small section at a time).
Second, posters should be understandable when the presenter isn’t there to explain it. Slides are almost always intended to accompany a speaker, and explicitly serve as aids to a presentation. You never walk into a room at a conference where there’s just a series of projectors, with slides flicking by on auto advance.
Thus, a poster will always have more elements to consider than any one slide. This presents a bigger challenge to layout, and causes greater planning demands.
One of the biggest struggles in designing a poster is to decide how much can be removed without making it impossible for someone to understand the poster when the presenter isn’t around. Unfortunately, there are no easy rules of thumb to follow.