HDMi foundation is founded by companies who own the home theatre environement (mainly movie conpanies and television) who puts DRM on HDMI to make it harder to illegally copy content like movies, ao they will always want to be anti open source because thats the request of streaming services/movie businesses. Its why for example, mobile devices have widevine levels. those levels basically determine how "unlocked" the device is and services will refuse to offer full functionality to unlocked devices because of it, be it audio or video.

Members of VESA, who control the displaypprt standard are generally computer companies are mostly not in the business of media, so they value specs over drm on changes, which for example a use case is that displayport allows for daisychaining diaplays.