Ballade as sung poem
Ballade was one of the formes fixes that was “particularly important in the 14th century.” Like the other two formes fixes, the ballade contains “complex repetition patterns with a refrain and music in two main sections” (ouvert and clos). In the late 15th century, the ballade and the other two forms “were abandoned by composers, through traces … can be heard … through the first half of the 16th century.”
The form typically consists of a a poem containing three stanzas, each of which is accompanied by music with the pattern I-I-II; such pattern can also be traced to 12th and 13th centuries (@wilkinsBallade2001). Like its etymology indicates, the ballade form was originally meant for dancing (@wilkinsBallade2001).
In the late 14th century, major poets began to stop setting their poems to music perhaps due to music’s increasing complexity; the ballade form specifically has also declined compared to the rondeau in the early 15th century (@wilkinsBallade2001).
Ballade as an instrumental piece
When used to describe an instrumental piece (usually piano), the term “ballade” suggests that the piece is in a narrative style. Its first use was by Frédéric Chopin in his Ballade No. 1 in g minor op. 23 (@brownBalladeIi2001). Chopin’s ballades have a structure based on programmatic / literary intention instead of a formal musical structure, though Chopin had not affirmed that the ballades were based on an actual poem (@brownBalladeIi2001).
In my opinion, the departure of ballade as a music from ballade as sung poetry is not necessarily negative. The development of music was known to be directed in increasingly complex ways when poets stopped setting their poems to music @wilkinsBallade2001. For the same reason, a composer in these times might have also come to dislike the tight coupling of lyrics and music when writing ballades, as such a coupling may limit his creative freedom. As such, the separation of the musical and poetical forms can be seen as beneficial to both poets and composers alike.