You can't compare De Blob to Rosalina's storybook because they were made to tell two completely different stories.
De Blob was meant to be a humorous adventure story about a blob and four rebels fighting against an evil organization. It was there to entertain, and lightly give some insight to the town they were trying to colorize in the process.
Roaslina's storybook was meant to be like a storybook from start to finish. You cannot get alot of explanation out of pictures from a children's storybook, you also need words. The words, combined with the picture, give it a purpose for why that picture is there. Despite being un-animated pictures with yellow text, they had alot of emotion. The last few chapters were, in my personal opinion, touching. The wonderful Comet Observatory, along with Rosalina's existance, with wonderous, but full of longing and heartbreak at the same time.
And it was just an extra. That's alot to accomplish in a side-element in what is considered the best 3-D Mario game.