In case you hadn't noticed, I've been attempting to reconstruct the history of the resurgence of Calvinism among younger evangelicals in the late 20th century, and I've been attempting to do this even in the order of these posts. So I'm not suggesting that the first (or the tenth) reason I'll give is the most important. Rather, I'm suggesting that in the 1940's there was little encouragement, though there were Spurgeon re-prints. Then there was added the increasingly known preaching of Lloyd-Jones. To that, by the late 1950's, you could add the re-prints of Banner of Truth. Then, in the 1960's & 1970's, I have suggested that the rise of Evangelism Explosion was quietly undermining one of the main objections American evangelicals had to a Calvinistic soteriology.
As we move into the 1970's and 1980's I would suggest that another main cause for the renewed popularity of Calvinism came through the Inerrancy Controversy. Controversy over the authority of Scripture has always been there. From the early church to the Reformation, various challenges to Scripture's authority were met and defenses erected. From the rising deism inside "Christian" countries in the 17th and 18th centuries, to the early work of Biblical critics in the 18th and 19th centuries, Bible-believing Christians have had to articulate their understanding of God's infallible working through sinful humans to compose His perfect Scriptures. From Gaussen in Geneva to Warfield in Princeton, the 19th-century churches produced careful defenses of the inerrancy of the Bible.
Controversy over the Bible has always been with us. But it is the storm summarized and energized by Harold Lindsell's 1976 Battle for the Bible that I specifically have in mind. (See the 9Marks website under articles for an annotated bibliography on this issue.) The Lutheran Church (Missouri Synod) was already in the depths of the storm by this time. The Southern Baptist Convention was just entering it. And evangelicalism at large became galvinized by the International Council on Biblical Inerrancy. Many of the most stalwart defenders of inerrancy were not Calvinists. But many were. Through this controversy, Jim Boice, RC Sproul, Jim Packer, Carl F. H. Henry, Roger Nicole and many other Calvinistic theologians were given larger audiences, especially among ministers. Old Princeton (especially the Hodges, Warfield & Machen) was re-introduced to a new generation.
But there was more to it all than young ministers beginning to read a Hodge here and there, or Carl Henry's vast project (God, Revelation and Authority in 6 volumes!). Theology was being discussed. Young evangelicals were encouraged not simply to preach and pray, visit and counsel, but to engage in theological thinking, to argue systematics. And not only that, but the very shape of the arguments used to promote inerrancy were exemplary of the Reformed understanding of God's complete and ultimate sovereignty over the completely responsible action of human agents. Much more could be said, but you get the idea. In the 1970's and 1980's, many young ministers were being educated theologically by theologians who had Calvinistic soteriology and a Reformed understanding of God and of His work with humanity. Part of what has led to Calvinism among the young has been the defense of Biblical inerrancy--in having a theological conversation at all, but especially by who was defending it, and how.