Poetry, with regular metre and rhyme is, in many ways, easier to set to music. Phrase length and rhythmic stress in the poem, for example, suggest the same elements in the song even if the composer intentionally contradicts the poet's intentions.
Prose texts don't have those imbedded characteristics which poetry and song share. The composer has to deal with irregular metre and stresses, phrases of different and irregular lengths, and the simple fact that prose must always "make sense" grammatically and that too, must be evident in the vocal part.
The composer should see that the melodic material holds up beyond simply rendering the words in an understandable fashion. Otherwise, one is simply writing recitative or chant. These are absolutely fine, of course, as a part of a longer piece, but not very interesting on their own (unless they're in a devotional piece). All of this has to be considered before and during the composition of whatever other music supports the voice or voices.
I first set prose in my piece for soprano and strings, Song of the Beloved. The text is drawn from the Song of Solomon in the King James Version. Yes, it is poetic but it's not poetry in spite of the beautiful Jacobean language. It was in writing this piece that I devised a process that I have applied again in other works.
More recently, I reworked some English translations of old Chinese and Japanese Zen poems and set them to music (6 Zen Lyrics). The texts, whatever their Asian language source poetry might be like, are blank verse, and present many of the same challenges to the composer as does prose.
Finally, here's the recording of my setting of a passage from the Gospel of Luke, again in the King James version. The children heard learned it without much difficulty because the text at once reflects the rhythm and contour of the words, and also provides a singable melody.
