I don't want to keep adding to the comments about why not X so I'll ask a more generalized question, what other options did you consider and why did you abandon them?
We looked at PureScript, ClojureScript, LiveScript, CoffeeScript, Dart, and compile language X to JS tools. We wanted static typing for run time correctness with respect to accessing data loaded from a server so that ruled out the dynamically typed variants like CoffeeScript. We also did not want to force our developers to learn a new language so that really only left TypeScript. At the time we were investigating, Flow by Facebook and AtScript by Google were not out yet. It seems like TypeScript will play well with Flow which is nice.