QML is optimized for writing the kind of fluid UIs that mobile applications favor today, meaning there is a lot of support for animations and other eye candy, and everything is heavily optimized to run smoothly on mobile devices. HTML, on the other hand, is not optimized for writing such user interfaces. So, writing a non-trivial, non-web-page-like user interface takes much less time to do in QML than in HTML (if it is possible to achieve in HTML at all) and the resulting user experience will be much better.
Of course, if your main concern is portability across mobile platforms, then HTML (and something like PhoneGap) is the way to go. Or, like a (fellow) Nokia employee put it: "If you want to go fast, use QML, If you want to go deep, use Qt C++. If you want to go wide, use HTML."