why do you think coleridge wrote this poem in the ballad form?

A ballad isn't just a song you can sing to with your headphones on. It tells an often complex narrative story filled with different rhythms and flow. At times this resembles a lyrical ballad but really it is an epic story at heart. By using quatrains, different meter and accented/unaccented syllables, Coleridge is able to give us a ballad reminiscent of ancient sea songs yet an interesting story.