That's because there are two main problems with the language when creating large Web applications.
The Need for Web App Speed
In fact most developers will probably never use asm.js directly at all. More likely they would use C or C++ (or take existing C or C++ applications) and then compile the code into asm.js using the Emscripten transcompiler.
Large Scale Web App Development
Web App Speed and Scale
It's an ambitious goal, so what is Dart? It's an open source programming language that's been designed with the application of large scale applications and high performance in mind. In fact calling Dart a language may be a misnomer, because a validator and various other development tools are all bundled in Dart.
Killer Dart Apps
As yet there are no plans for any other browsers beyond Google's to support Dart directly, so the chances of it becoming the lingua franca of the Web appear slim. But Buckett maintains that the twin advantages of performance and ease of large scale development -- plus the ecosystem of Dart tools that are available -- may mean it proliferates.