Finally, I found an answer to my question in Stephan T. Lavavej's blog post C++11/14 STL Features, Fixes, And Breaking Changes In VS 2013:
The VS Update mechanism is primarily for shipping high-priority bugfixes, not for shipping new features, especially massive rewrites with breaking changes (which are tied to equally massive compiler changes).
Major versions like Visual C++ 2013 give us the freedom to change and break lots of stuff. There's simply no way we can ship this stuff in an Update.
Q5: What about the bugfixes? Can we get those in an Update?
A5: This is an interesting question because the answer depends on my choices (whereas in the previous question, I wouldn't be allowed to ship such a rewrite in an Update even if I wanted to).
Each team gets to choose which bugfixes they take to "shiproom" for consideration to be included in an Update. There are things shiproom won't let us get away with (e.g. binary breaking changes are forbidden outside of major versions), but otherwise we're given latitude to decide things. I personally prioritize bandwidth over latency - that is, I prefer to ship a greater total number of bugfixes in every major version, instead of shipping a lesser total number of bugfixes (over the same period of time) more frequently in multiple Updates.