There was a strong and long debate about the pros and cons. One of the strongest argument against was this here, including interoperability. I believe Cory Doctorow has also written multiple articles against DRM. I agree that DRM is generally bad.
However, the arguments for implementing it were the following: at least two other browsers will adopt the tech and that will allow it (per W3C process) to graduate from a Candidate Recommendation to a full Recommendation, making it part of the web platform.
On top, what would it mean for marketshare if people had to install a second browser just for watching movies? Would the non technical users accept and understand this without switching to a browser that they’d consider "actually feature complete"?