Speaking for myself, the key selling point for Typescript (and Flow) is its pragmatism. While it might not have the soundness of a Bucklescript or Purescript, it does what its alternatives have chiefly failed to do for me: Allow me to see immediate incremental benefits without having to refactor an entire codebase.

While the alternatives might be in some ways a better start a greenfield project in, the ability to gradually add types to an existing codebase with developers who are just learning the language with the ability make the system sounder over time is a killer feature for me. Also, the big-company backing helps corporate slaves like me trying to sell a new technology to management.