Many aspects of today’s shallow American hymnody are rooted in the 19th-century revivals. This is a huge topic, of course, but to get a little glimpse I like how George Marsden writes about it in Fundamentalism and American Culture.
“The surge of revivalism associated with the rise of Charles Finney in the 1820s which developed in the ‘New School’ tradition certainly did not forsake intellect, but it did create new channels for emphasis on emotion throughout American evangelicalism. Sandra Sizer in her analysis of the rise of the gospel song in nineteenth-century America has suggested that Finney’s revivals marked the beginning of the attempt to build a new Christian community united by intense feeling. The focal point for the emphasis was the ‘social religious meeting,’ small groups gathered for prayer, Bible study, witnessing, and song. Witnessing, or testifying to one another about how God had transformed their lives, was an important way in which these communities built themselves up and provided emotional support.”
“Finney added emphasis on such meetings to his more-or-less conventional mass-preaching services, but by the time of the remarkable businessmen’s revival of 1857-1858 the awakening itself originated in noon hour prayer meetings which were just such ‘social religious meetings.’ Every new evangelical movement of this entire area, through the rise of fundamentalism and including the holiness, pentecostal, and premillennial movements, had a base in some form of ‘social religious’ gathering.”
“The revivals of Moody and Sanky, Sizer argues persuasively, in a sense applied to the principles of the smaller group meetings on a massive scale. The use of a song leader, which Sankey made a lasting part of evangelicalism, was a conspicuous means of building emotional ties. The most common theme was the distress of sin, to be relieved by a passionate surrender to the incredible love of Jesus. Hymns that told stories of prodigals reclaimed and the like made the song itself a kind of witnessing.”
“In contrast to eighteenth-century hymns like those in the influential collection of Isaac Watts, the focus of revivalist songs shifted from praise of the awful majesty of God and the magnitude of his grace revealed in Christ’s atoning work, to the emotions of those who encounter the Gospel. Similarly, Moody’s sermons virtually abandoned all pretense of following conventional forms of explicating a text, and were closer to ‘layman’s exhortation’ filled with touching anecdotes with an emotional impact comparable to that of personal testimony.”
There is more to it, but these are some of the theological, historical, and practical reasons why confessional Reformed churches typically do not sing these songs. In other words, we avoid these songs and worship techniques for several different reasons and not just to be “traditional” or “conservative.” I recommend Marsden’s book Fundamentalism and American Culture if you want to dig deeper into hymnody and other aspects of American Christianity.