A desire to not have IP coupled to Microsoft and a desire to put the burden/discipline on the self to ensure clean API design versus using TypeScript as a crutch to do it for you. In my experience, writing JavaScript code that's conscious of types/inputs/etc. is far easier (and more productive) than maintaining a TS codebase and all the trimmings that come with it (i.e., dependency on build tools, frameworks, etc). A lot of hidden costs.