Because Bun is still far less mature (and the stack on which it builds is even less so - Zig isn't even 1.0).

Because its Node.js compat isn't perfect, and so if you're running on Node in prod for whatever reason (e.g. because it's an Electron app), you might want to use the same thing in dev to avoid "why doesn't it work??" head scratches.

Because Bun doesn't have as good IDE integration as Node does.