1) The United States' nuclear arsenal: The United States possessed a significant advantage in nuclear weapons over the Soviet Union, and the threat of nuclear retaliation deterred the Soviets from taking any action that could lead to a nuclear war.
2) The strength of the NATO alliance: The North Atlantic Treaty Organization (NATO) was a military alliance formed by the United States, Canada, and Western European nations to counter the threat posed by the Soviet Union. NATO's collective security guarantee meant that an attack on one member state would be considered an attack on all, and the Soviet Union was unwilling to risk a confrontation with the entire Western alliance.
3) The economic and political cost of invasion: Invading Western Europe would have been a costly and risky undertaking for the Soviet Union, both economically and politically. The Soviets would have had to contend with the resistance of Western European countries, and the invasion would likely have led to economic sanctions and other negative consequences for the Soviet Union.
In addition to these factors, the Soviet Union was also facing internal challenges, such as economic stagnation and political unrest, which made it less likely to take on the additional burden of invading Western Europe.