I would guess that compatibility with existing JS libs is far easier with Typescript. You can trivially consume an existing JS lib from TS or export/compile your TS lib to pure JS, which other JS then can seamlessly consume.
Although I don't have bigger experience with Elm, Purescript, Scala.js, Ceylon, Websharper & Co I guess its more complicated with them. An advantage here is also that Typescript does not bring along it's own standard library (e.g. new collection types) which could cause problems on interoperability.
Another advantage is getting other people on board. Getting developers from pure JS to typescript is not difficult, especially when they are used to Java/C++/C# (which most developers are). The learning curve for the other languages will be higher for most developers.
