Largely because of the fear of nuclear war- remember the SAD policy? That kept both the USSR and USA from making the first strike. the USSR's conventional military was slightly better than Putin's but still could not have supplied an invasion force into Europe. Of course, the USSR and many US politicians would have people believe otherwise but the US military knew better- and keeping people afraid of the big, bad Bear was great for padding the budgets of arms manufacturers. If the truth had ever become known, the Pentagon would never have received the massive budget it got then that has since only grown larger.