Why do you think that Wordsworth worte"we are seven"in the form of a ballad?

I'm not sure it is a ballad. The poem is composed of sixteen four-line stanzas, and ends with one five-line stanza. Each stanza has an abab rhyming pattern. Wordsworth has noted that he wrote the last line of this poem first, and that his good friend Samuel Coleridge wrote the first few stanzas.