The existing plugins are heavy and don't integrate well with other web technologies. In addition, the NPAPI interface is not standardized (I believe). Thus, the point is to narrow down the scope of plugins from everything that Flash and Silverlight do to just a piece of code focused on DRM. Everything else that people use Flash and Silverlight can be replaced by standards and directly implemented in the browser. However, DRM, by its nature depends on closed-source compiled binaries, which means that it has to remain in a plug-in.