I am familiar with this text because it was set to music to music by English Composer John Tavener in 1982, You need to look at the larger context of this poem in reference to the lamb, and the Lamb (although both references are sometimes capitalized). Pay close attention to the lines "He is called by thy name, For he calls himself a Lamb..." (read on). If you understand these lines, I believe it unlocks the answer to why this poem is included in Blake's "Songs of Innocence, and his choice of text and style. I hope this is helpful.
