First, there is no standard for WebM bandwidth-adaptive streaming yet, over HTTP or any other protocol. Secondly, a quote from that site:
One word about the adaptive bitrate mechanism on WebM : contrary to other OTT technologies, adaptive bitrate settings are controlled on the server side only...
So, it holds state on the server, meaning it requires custom server software, and therefore doesn't scale cheaply. The beauty of the streaming-over-HTTP solutions is that you can take advantage of the huge number of caches in CDNs, ISPs, and corporations for free. We did some testing, and around 75% of our visitors are behind some form of caching proxy (we're a B2B company). That means as much as 50% of our video bill can be covered for free by our customers if we switch to an HTTP based streaming solution. Note that progressive download over HTTP doesn't get high hit ratios, because many (most?) caches have some default maximum file size limits to prevent one user from blowing out the cache. HTTP-based streaming, with its smallish (10-second) chunking, solves that problem.