I could not even begin to guess why Bahá'u'lláh wrote this epistle, but it seems that He set the record straight of what happened at the time by the followers and the enemies of the faith and then showed how great Gods forgiveness is.This is certainly true. I would like to add a little to this by looking at Mary's question in the context of the period and events leading up to 1891. At the time the Epistle was written, the distinction between Bábís and Bahá'ís was not well known or understood. But there was a great distinction between the Bábís of the 1850s and the Bahá'í community created by Bahá'u'lláh. That distinction was because of Bahá'u'lláh's transformative power and the Epistle to the Shaykh was written to point out this great change.
In the Sermon on the Mount Jesus said: "Beware of false prophets, which come to you in sheep's clothing, but inwardly they are ravening wolves." (Matthew 7:15) Isn't it interesting how Bahá'u'lláh could so eloquently use the same metaphor that Jesus used, showing us, if nothing else, that the Manifestations of God and their followers have suffered the same kinds of opposition and persecution. I would be interested to know the similar reference from the Hebrew prophetsThere are two Hebrew Prophets who use the term. The pre-exile Prophet Zephaniah writes in His third chapter of the corruption of Jerusalem, saying, "Woe to her who is rebellious and polluted, To the oppressing city! She has not obeyed His voice, She has not received correction; She has not trusted in the LORD, She has not drawn near to her God. Her princes in her midst are roaring lions; Her judges are evening wolves (Zeph 3:1-3). Similarly, in the Book of Ezekiel, the exile Prophet Ezekiel, in his twenty-second chapter speaks out against the sins of Jerusalem and Israel's corrupt leaders, saying "Her princes in her midst are like wolves tearing the prey, to shed blood, to destroy people, and to get dishonest gain." (Ezek. 22:27)
I have found myself wondering why the last significant work of Bahá'u'lláh is an epistle to a malicious enemy of the faith why Bahá'u'lláh did not simply finish his series of books of Divine Revelation with a compilation or summation or some other finalizing type of work written expressly for the believers.This is a great question and your answers were very interesting and thoughtful. Its a hard question too. Why did this Epistle end up being the last outstanding Tablet revealed by the pen of Bahá'u'lláh? There is a transcendental aspect to this question we probably can't ever answer, but in God Passes By Shoghi Effendi gives two basic reasons Bahá'u'lláh revealed the Epistle:
"At this moment a Voice was raised from the right-hand of the Luminous Spot: 'God! There is none other God but Him, the Ordainer, the All-Wise! Recite Thou unto the Shaykh the remaining passages of the Lawh-i-Burhán (Tablet of the Proof) that they may draw him unto the horizon of the Revelation of his Lord, the God of Mercy, that haply he may arise to aid My Cause..." (pp. 96-7)