WebGL, O3D or [your favorite 3D-JS lib] are made by gfx-coders for gfx-coders who know how to deal with a 4×4 transformation matrix and speak GLSL as their first language. Most web-page developer have a different profile. They just would like to build their 3D-UI, Visual-Analytics or Web-Shop application and want to utilize some 3D technology.
If you build the next high-end browser-based online game or visualization demo then use WebGL or O3D. But if you simply need some 3D elements in your Web-App then try X3DOM.
Collada is really designed as interchange and intermediate format to transport and manage your 3D data. The Collada specification does not include, unlike X3D, a runtime or event model, which is needed for per-frame updates on the 3D-side (e.g. animations). Therefore this project uses a well defined subset of X3D (called Profile in the X3D-World) for X3DOM. For more background information about how Collada and X3D relate and why “X3D is ideal for the Web” please read the Whitepaper by Rémi Arnaud and Tony Parisi.
Q: Why JS? Can you not write the system/plugin in C++, Java, …
Well, the developer of this project worked on different native commercial and open-source X3D-runtimes before. The limitations of the current plugin interface (e.g. how to monitor DOM-changes) would make a implementation hard or even imposible.
Q: Why any standardization and native implementation if there is already the X3DOM-JS runtime?
Short answer: Feature and Speed. The current JS/WebGL layer does not allow to implement e.g. spatial Sound or specific image loader and although WebGL is an impressive step forward we still have to do all the scene management and updates in JS.