There are still so many features that had been introduced there (client-side includes with fallback, “everything is a link”, etc) that are still sorely missing 8-(
I wonder if there was also a growing dislike for XML in general behind this choice? (hurr durr namespace confusing). It's ironic that we have to thank MS for pushing for the little support of XML in browsers we still have (and they are now working on removing 8-/).