What changed was that “web 2.0” happened. Web design’s focus changed from documents to applications.
CSS Zen Garden was about styling “web 1.0” HTML documents.
But dynamic UIs have completely different styling needs. People started building JavaScript applications, and they started needing UI layouts similar to those provided by desktop UI frameworks. And CSS was never designed for that. Concepts like “float” were entirely derived from the word processor universe. CSS had to add several complex new layout engines over the following decades.
CSS Zen Garden was about styling “web 1.0” HTML documents.
But dynamic UIs have completely different styling needs. People started building JavaScript applications, and they started needing UI layouts similar to those provided by desktop UI frameworks. And CSS was never designed for that. Concepts like “float” were entirely derived from the word processor universe. CSS had to add several complex new layout engines over the following decades.