Anthropic only acquired Bun in December of last year. They weren't there in the first place, to make the decision.

Zig has some advantages for such projects, especially in the beginning.

Among them:

- much easier to iterate on (due to the language being simpler and compilation much faster)

- native C/C++ interops (Zig can compile C and C++ and mix it with Zig) which is crucial for a node-replacement runtime that runs an open source JS engine

- fewer dependencies and trivial static linking

I guess that now that they've been acquired by Anthropic there's this combination of having both in-house Rust talent, AI which does better on Rust, and the funding and resources necessary to undertake such a migration.