T. David Gordon lists 8 reasons the change is happening:
1. Contemporary worship music hymns not only were/are comparatively poor; they had to be. One generation cannot successfully “compete” with 50 generations of hymn-writers; such a generation would need to be fifty times as talented as all previous generations to do so. If only one-half of one percent (42 out of over 6,500) of Charles Wesley’s hymns made it even into the Methodist hymnal, it would be hubristic/arrogant to think that any contemporary hymnist is substantially better than he. Most hymnals are constituted of hymns written by people with Wesley’s unusual talent; the editors had the “pick of the litter” of almost two thousand years of hymn-writing. In English hymnals, for instance, we rarely find even ten of Paul Gerhardt’s 140 hymns, even though many musicologists regard him as one of Germany’s finest hymnwriters. Good hymnals contain, essentially, “the best of the best,” the best hymns of the best hymnwriters of all time; how could any single generation compete with that?
Just speaking arithmetically, one would expect that, at best, each generation could represent itself as well as other generations, permitting hymnal editors to continue to select “the best of the best” from each generation. Were this the case, then one of every fifty hymns we sing should be from one of the fifty generations since the apostles, and, therefore, one of every fifty should be contemporary, the best of the current generation of hymnwriters. Perhaps this is what John Frame meant when, in the second paragraph of his book on CWM, he indicated that he had two goals for his book: to explain some aspects of CWM and to defend its “limited use” in public worship. Perhaps Prof. Frame thought one out of fifty constituted “limited use,” or perhaps he might have permitted as much as one out of ten, I don’t know. But our generation of hymnwriters, while talented and devout, are not more talented or more devout than all other generations, and are surely not so by a ratio of fifty-to-one.
2. Early on in the contemporary worship music movement, many groups began setting traditional hymn-lyrics to contemporary melodies and/or instrumentation. Sovereign Grace Music, Indelible Grace, Red Mountain Music, Reformed Praise all recognized how difficult/demanding it is to write lyrics that are not only theologically sound, but significant, profound, appropriate, memorable, and edifying (not to mention metrical). If the canonical Psalms are our model, few hymn-writers could hope to write with such remarkable insight (into God and His creatures, who are only dust) and remarkable craftsmanship (e.g. the first three words of the first Psalm begin with the first letter of the Hebrew alphabet, aleph (א), each also has a shin (ש), and two of the three also have a resh (ר), even though each is only a 3-letter word. Even those unfamiliar with Hebrew cannot miss the remarkable assonance and alliteration in those opening three words: “ashre ha-ish asher”).
3. As a result, the better contemporary hymns (e.g. “How Deep the Father’s Love,” “In Christ Alone”) have been over-used to the point that we have become weary of them. These two of the better contemporary worship music hymns are sung a half-dozen times or a even a dozen times annually in many contemporary worship music churches; whereas “A Mighty Fortress” may get sung once or twice (if at all); but neither of the two is as good as Luther’s hymn. What is “intrinsically good” (to employ Luther’s expression about music) will always last; what is merely novel will not. Beethoven will outlast 50 Cent, The Black Eyed Peas, and Christina Aguilera. His music will be enjoyed three hundred years from now; theirs will be gone inside of fifty years.