From a practicality standpoint, writing in TS allows you to execute arbitrary JS in-process with a simple exec() call; I have no doubt they'll use this to more deeply integrate agents with existing codebases in the near future. E.g. agents rather than just reading the code, will be able to directly import the data structures themselves, use libraries within the JS/TS ecosystem to parse code into an AST, and execute in-process test harnesses to validate behavior while editing.

And the MCP field is already pretty heavily saturated with TypeScript and JSONSchema, so using TS for it is a very ergonomic experience. Also since it's written in TypeScript, it's much more easy to integrate it with editors like VSCode (or Google's new Antimatter) which are built on top of Electron.