Because WebRTC is not finalized and I think Microsoft has taken an approach of not implementing the experimental features.

You can get the prototype WebRTCn as a plugin for IE through Microsoft's Interoperability Bridge here: http://html5labs.interoperabilitybridges.com/prototypes/cu-r...

A lot of browsers certainly chrome implement experimental features with vendor prefixes just to get high up in html5 tables such as those.