To allow for the widest possible cross-browser app compatibility today. Dart currently support all modern browsers (back to IE9) - forcing users to use Chrome would divide the web, which is the last thing we want to do. That said, we're working on getting the VM into Chrome, but really that comes down to an added performance boost. We expect most developers will target JS to ensure cross-browser compatibility. It's entirely possible (encouraged, even) to deploy both.
The last thing I want to do is write in one language, compile it to another, then need to debug across language boundaries.
The good news is that you can write and debug purely in Dart and test in Dartium (Chromium + DartVM) with no compile step today. The dart2js step is a deployment step. Obviously, you'd want to do the usual testing of the final compiled output in a few browsers just like you'd do with a JS app today, but for development you can live entirely in Dart.