Atom doesn't have built-in support for TypeScript annotations. I've gotten the impression that VSCode is somewhat more popular with the TypeScript crowd.
Since MS develops TypeScript they have a big advantage in making TypeScript a first class citizen in VSCode. I see many developers who would prefer a different IDE normally, but decide on VSCode because it seems to be ahead of everyone else on TypeScript currently.
Not sure that applies like that. Typescript support is based on the Typescript language server, which uses their open source language server protocol, which is open for other editors to use (and indeed adopted by a bunch of them). So in theory other IDEs could support TS with the same level of VSC as long as they adopted the protocol, without the need for discussion TS functionality.