couple of reasons. one big one is flexibility. angular2 (er, @angular) runs on client, server, native (react native/nativescript) and we hope many other platforms in the future. this lets us packages for common dependencies for various platforms, as well as enabling us to not ship the template compiler for precompiled apps.

the other is providing a single entry point to each of those packages, which makes it easy to configure for rollup/webpack2 etc when you want to use the ES6 source over the CJS.