Why did the development of more effective nuclear weapons ultimately make direct confrontation between the United States and the U. S. S. R. less likely?

They were all afraid. They had more and more weapons but were more and more afraid because they knew that if any of them attacked, the others would retaliate and that monstrosities would be committed. They had many nukes but nobody used them out of fear of what the other one would do since they wouldn't be able to defeat the opponent before he launched his nukes.