Even backers of the current WebRTC specification acknowledge that the SDP problems have so far proven intractable. The WebRTC spec hinges on getting SDP fit for purpose.
Microsoft's PR official reasons in their own words.
The real reasons may or may not be related. Personally, I don't trust any statement from Microsoft about improved functionality or specification. That might be right in this case, but their past actions indicate that they care about market dominance rather than any technical detail.
Microsoft's reasons in Microsoft's own words:
http://blogs.msdn.com/b/interoperability/archive/2012/08/06/...
The Ars article sums this up as:
Even backers of the current WebRTC specification acknowledge that the SDP problems have so far proven intractable. The WebRTC spec hinges on getting SDP fit for purpose.
So the title of the article is a bit link baity.