For the same reason that Flow's dedicated type annotation syntax is the standard. It's just easier to use. This change will effectively make using ts as easy as using Flow, which is a huge win.
Interesting. From my perspective using Typescript and its transpilation power entirely instead of Babel is even easier still. There's a lot less complexity in my builds if I don't need to manage Babel. From that perspective, Typescript has always been easier than Flow, too, though some of that is also bias from how hard Flow has been to get working on Windows up until somewhat recently. I realize though that there are many perspectives on this issue and I asked the previous question because I was indeed curious about your perspective, thanks.