Widevine is currently in the Chrome, Brave, and Firefox browsers. What are the odds we'd have gotten that (instead of a Chrome DRM solution, Firefox DRM solution, and Brave DRM solution) without the EME standard?
The w3c spec can be expected to put as small a subset of the client stack as possible into the black box, whereas a proprietary API would try to pull in as much as it can, aiming to eventually usurp the entire client. Remember silverlight?