mainly because we decided to fight on two fronts at once. If the people who have been slowly on the retreat in Iraq for the last year or two had spent that time in Afghanistan instead, we might have had the manpower to do one job properly. Instead we're failing in two jobs.
I have no idea where the arms are coming from but remember the Pentagon mislaid something like 200,000 weapons in Iraq. No need to guess where they went; but some may well have found their way onward to Afghanistan.