Why were poems from The Book of Songs often set to music?

Poems from The Book of Songs (Buch der Lieder) by Heinrich Heine were frequently set to music because they provided a high-quality repertory of lyric poetry that resonated with 19th-century composers.12

During this period, the development of the German lied was driven by a reaction against the superficialities of operatic arias and an increasing emphasis on the expressive potential of the piano as an accompaniment. Composers such as Robert Schumann and Franz Schubert were drawn to Heine’s work due to their sensitivity to the essentials of the text and their ability to combine musical design with the evocative, bittersweet, and self-ironic nature of his verses. Heine’s poetry offered a depth of emotion and literary quality that allowed composers to create intimate, nuanced art songs where the vocal line and accompaniment were closely linked.23