There’s also an FAQ discussion about the language choice: https://github.com/microsoft/typescript-go/discussions/411

TL:DR;

- Native executable support on all major platforms

- He doesn't seen to believe that AOT compiled C# can give the best possible performance on all major platforms

- Good control of the layout of data structures

- Had to have garbage collection

- Great concurrency support

- Simple, easy to approach, and great tooling