The speaker of "On Being Brought from Africa to America" is probably a woman, although we never know either way. We can never assume the speaker is the same person as the poet, but since the title of this poem is about being taken from Africa to America, and we know Phillis Wheatley was taken from Africa to America, we can at least guess that this poem is somewhat autobiographical.
So, if that's the case, why doesn't the speaker sound like an uneducated woman who was kidnapped into slavery? What's with the formal poetry and rhymed couplets?
Well, Wheatley was taken as a slave (dislike), but the Wheatley family treated her as one of their own and gave her a formal education (like). So she could read English, Latin, and Greek as a child. Not bad, right? And she was also writing her own poems. That brings us, finally, to our point—this speaker sounds like a calm, collected, educated person making an argument. Blah! How boring!
But, actually, the speaker is echoing the neo-classical sentiment of restraint and balance. Think of a well-reasoned philosopher laying down the cold hard truth about racial equality. Rather than getting up and reciting a passionate speech (think Martin Luther King Jr.), Wheatley is making a calm, collected statement based on her experience and transition from being a child in Western Africa to an educated poet in America. The strength of her voice comes in its calculated delivery of each line and how it's neatly bound with rhyme throughout the poem.