They're much harder to "disassemble" and applications written in them are much harder to modify and a relatively open DOM + JS model. Looks like a very promising technology for an ad company that's been pushing to make ad blockers significantly less effective, and has never let you install an adblocker into the most popular browser on the most popular platform in the world.
In the case of Google Docs kt might be because it is really convenient to use canvas because then it gets much harder for clients to do things Google don't like, i.e. interoperability.
In other cases, like in IE^h^hChrome and large Google products like Calendar and YouTube there have been some interesting cases I think.
