The DOM gives a developer very little control over when something should be repainted (or how repainting occurs) relative to the compositing options available when controlling one's own canvas. Moving the rendering engine to canvas allows the docs team more control over the optimizations of layout and rendering they can do (especially cross-platform; there are a hundred hundred mutually-incompatible bugs and quirks in Firefox, Chrome, IE, etc.'s layout and content rendering algorithms that make cross-platform high-performance very hard to guarantee at the DOM layer of abstraction).