New Immigration Bill MIGHT Have Prevented Boston Bombings But...
Quite simply, this is a problem that was identified long ago and still remains a problem. Adequate tracking of those here on visas is something that needs to be addressed now all on its own and doesn't need to be part of a larger comprehensive immigration law. The Department of Homeland Security was created for just such a thing and has proven only that adding a massive layer of bureaucracy rarely solves the problem.
The fact that one provision of a bill might be functional isn't a case for passing the entire thing. There's also a presumption by those advocating for new laws and regulations that everything will be properly implemented and functional. When it comes to immigration, there's a long history of the government not availing itself of existing laws that could address the problem.