I worked on the Basic Workstation team; my part was the desktop and folders code. As I recall, the whole BWS rewrite was the result of Robert Ayers coming along and trying to rewrite the bottom layers of Star from scratch. The framework he came up with became the basis for BWS.
Traits was the object oriented multiple inheritance archetypes layered on top of Mesa that Star was based upon and required a static analysis step of all objects in the system to optimize object layouts—this is why it was a closed system. After the changeover to BWS, a few years later, only the document editor, and all object types represented in documents (graphics, tables, equations…) continued to use traits.