Atom/GitHub wasn’t owned by Microsoft when Visual Studio Code gained traction. I’m guessing that the maintainers had no interest in giving Microsoft more help in reducing Atom to an unmaintained status and promoting Visual Studio Code for free. If the maintainers work for Microsoft now, I highly doubt it was ever a choice to move on. Teletype was probably in progress as well pre-acquisition, as that sort of feature came much much later in Visual Studio Codes development.