I wouldn't have enjoyed the story as much if we knew all along. It's already obvious that Luffy is the "chosen one" - he's the protagonist of a shonen manga. It's a huge responsibility which needed to be obscured in order to enjoy their light-hearted adventures without pressure. I know I'd be tapping my foot while the Straw Hats party it up.
Besides that, we needed to build the dramatic tension, otherwise it'd change the direction of the story from the beginning. On Day 1, if we were told Luffy ate the Nika fruit, why should we care? Who's Nika and why is he important? The Nika fruit's very existence is an advanced concept which only makes sense in context with later plot elements. It reveals too much about the nature of Devil Fruits too soon.
Imagine in the first chapter, trying to establish Devil Fruit powers for the first time, and then dropping this bomb. It's just too much weight on the story.