Why is Apollo the god of poetry?

Apollo was associated with music and art through his sisters, the Muses (who were also his lovers in the cases of :

Calliope Muse of epic poetry

Kleio the Muse of history

Terpsichore the Muse of choral song and dancing

Thaleia the Muse of comedy and bucolic poetry

Ourania the Muse of astronomy and astronomical writings

As well as Apollo's daughter Muses (by unknown mother/s) Kephiso, Apollonis and Borysthenis, or Nete, Mese, Hypate.