You have to remember that Dart Editor was started before Google moved to IntelliJ for Android (at least publicly). Dart Lang is a relatively small team compared to the people working on the Android dev stuff, so my guess is that it probably was a matter of not dividing resources for an unnecessary transition. In my experience Dart Editor works quite well. Of course that's just anecdotal... Perhaps a Google engineer can shed more light on this matter.

Dart is also supported by other popular editors

IntelliJ and Webstorm http://www.jetbrains.com/webstorm/webhelp/dart-support.html

Netbeans https://blogs.oracle.com/geertjan/entry/dart_and_netbeans_id...

There's also a Sublime text package on github.