I am currently reading Don Kistler's (pictured above) biography of Christopher Love; in the following extract, Mr. Kistler explains why this Puritan only sang the Psalms in worship:
D. Kistler, A Spectacle Unto God: The Life and Death of Christopher Love, p. 90Christopher Love was a Covenanter. One of the things that makes them unique is their commitment to exclusive psalmody, and their strict keeping of the Sabbath. In other words, Covenanters will sing only the divinely-inspired psalms of Scripture in their worship services, and they sing them without musical accompaniment. Since God wrote the psalms, they reason, why sing hymns written by fallible men?