Put yourself in the shoes of a browser dev faced with a torrent of angry users asking why you "broke Netflix". They won't listen to what seems like a wishy-washy response about the principles of "the open Internet", all they will care about is that they had Netflix and now they don't and it's your fault.
Devil's advocate, I'm no fan of DRM, but from the end user's point of view, it's a necessity.
If (in an ideal world) Chrome and Firefox had said "no, we aren't implementing it" then it falls back to Netflix to stop harming users. And they need to make business so they'll make a crappy SilverLight client that breaks every other week. It's not the browser developers' fault that a several million dollar company decided to build their business model around the need for DRM.
The biggest problem with taking the principled stand is that you run into the prisoner's dilemma. And every CS student can tell you that the game theory says you should always defect, if the other parties are adversarial.
I think the biggest problem in the "browser game" is that the different free browsers often see each other as enemies and not as friendly competition. It's a shame because monocultures are what almost killed the internet in the early days, and I don't understand why we're heading back to ActiveX.
In an ideal world nobody would want DRM. In the real world, Chrome and Firefox are not part of the same team, and never were. Google is a media distribution platform like Netflix (YouTube, Play, etc), and like them, they have zero interest in pissing off producers based on lofty ideals.
It boils down to what is more important for user: will they change browser to play DRMed videos or switch video provider to one without DRM.
Not supporting EME in browser with falling usage (Firefox) is likely suicide, but W3C doesn't make browsers and can take any ideological position without consequences.
Devil's advocate, I'm no fan of DRM, but from the end user's point of view, it's a necessity.