There's a very simple reason for focusing on the nuclear issue. Many, many issues are of supreme importance in one way or another, but if we blow ourselves up with nuclear weapons, no other issue is really going to matter. Quite possibly there would be no other human beings left to be concerned about anything else.
Unprecedented warnings by officials most closely linked with nuclear arms negotiations and defense strategy indicate that we are running out of time. If we fail to act soon, the scars of a major nuclear disaster will mark our immediate and distant future.