Why is Maharishi Valmiki incarnated as Shri Tulsidas in Kali Yuga? What was the meain reason behind that?
See, Valmiki muni was incarnation of Śrī Brahmā Ji who has himself written 100 crore Rāmāyan to sing the sweetest glory of Śrī Rām, as said by following scriptures;
वाल्मीकिना तु यत्प्रोक्तं रामोपाख्यानमुत्तमम् । ब्रह्मणाभिहितं यच्च शतकोटिप्रविस्तरम् ।।
(Matsya Purana, Chapter 53, Verse 71)
वाल्मीकिना च यत्प्रोक्तं रामोपाख्यानमुत्तमम् । ब्रह्मणा विहितं यच्च शतकोटिप्रविस्तरम्॥
(Skandh Purana, Prabhas Khand, Chapter: 2, Verse 95)
Overall meaning is that Śrī Brahmā Ji has written 100 crore Rāmāyana which Śrī Vālmīkī Munī has retold.
And when Lord Rāma appeared on Earth than Śrī Brahmā Ji gave order to Śrī Vālmīkī Munī to write ✍️ the beautiful character of Parātpar Par-Brahma Śrī Rāma. And Śrī Vālmīkī Munī followed the same and he wrote it down.
And now here is a twist;
As Śrī Ved Vyās has written whole Maha-Bharat, all eighteen Purān etc but he was not satisfied as he has shown magnificence character of Lord Śrī Krishnā but something was missing and that was sweet glories of Śrī Vrindavan Bihari Lal;
भवतानुदितप्रायं यशो भगवतोऽमलम् । येनैवासौ न तुष्येत मन्ये तद्दर्शनं खिलम् ॥ यथा धर्मादयश्चार्था मुनिवर्यानुकीर्तिता: । न तथा वासुदेवस्य महिमा ह्यनुवर्णित: ॥
(Śrīmad Bhāgwatam 1.5.8/9)
Śrī Nārada said: You have not actually broadcast the sublime and spotless glories of tge Lord. That philosophy which does not satisfy the transcendental senses of the Lord is considered worthless. Although, great sage, you have very broadly described the four principles beginning with religious performances, you have not described the glories of the Supreme Personality, Vāsudeva.
Exactly same condition occured with Śrī Vālmīkī Munī, he has written whole Vālmīkī Rāmayan keeping magnificence character (ऐश्वर्य परत चरित्र) as a center theme and missed the sweetest glories of Rajdulāre Śrī Raghunandan for which he again incarnated as Śrīmad Göswāmi Tulsidās Ji Maharāj in Kali-Yuga to complete his remaining work as Śrī Tulsidās mentioned this in Śrīmad Rāmcharit Mānas in Balkānd, Shloka 7;
नानापुराणनिगमागमसम्मतं यद् रामायणे निगदितं क्वचिदन्यतोऽपि स्वांत:सुखाय तुलसी रघुनाथगाथा भाषानिबंधमतिमंजुलमातनोति ॥ 7 ॥
In accordance to many Puranas, Vedas and Shastras and which is described in Ramayana and also available from some other places, Tulsidas composes the story of Raghunath in very beautiful language for the pleasure of my heart.
Here “स्वांत:सुखाय" तुलसी रघुनाथगाथा भाषानिबंधमतिमंजुलमातनोति in 7th shloka says clear that for own's satisfaction Śrī Tulsidās has written down whole Śrīmad Rāmcharit Mānas, eventually Śrī Tuslidās has completed the remaining work that was left in (24th) Treta Yuga means he has now completed the Śrīmad Rāmāyaṇa.
First of all, Shri Tulsidas ji had started writing Ramcharitmanas in Sanskrit only but when he woke up in the morning after resting at night, he used to see that the whole page has become blank. This process continued for a few days, he used to write Ramayana in Sanskrit and when he woke up in the morning everything was clear as crystal on page. Then after that Lord Shiva commanded to write Ramacharitmanas in Awadhi language. Then Śrī Tulsidās wrote it in Awadhi language because the social conditions at that time were such that the teaching of Sanskrit was completely eliminated because the Islamic invaders in North India had destroyed almost all the Gurukulas.
Now let's see what scriptures says about Śrīmad Tulsidās Ji Maharāj and about Śrīmad Rāmcharit Mānas;
वाल्मीकिस्तुलसीदासः कलौ देवि भविष्यति । रामचन्द्रकथामेतां भाषाबद्धां करिष्यति ॥
(Bhavishya Purana, Pratisarga 4.20)
In the Bhavishyottar Purana, after telling the entire story of Sri Rama, Lord Bhutabhavan Shankara says to Goddess Parvati – “O Parvati! Maharishi Valmiki will become Tulsidas in Kali Yuga and will translate this Rama Katha into vernacular (awadhi) language.”
वाल्मीकितुलसीदासः कलौ देवि भविष्यति । रामचन्द्रकथां साध्वीं भाषारूपां करिष्यति ॥
Shiv Samhita (Excerpted from Gita Press Gorakhpur Veda Katha Ank, Pg. No. 285)
Translation is approximately same that Śrī Vālmīkī Munī will incarnate as Śrī Tulsidās in Kali-Yuga to explain Rāmachandra Katha in vernacular language.
Sri Vaishnava Acharya Srimad Goswami Nabhadas Ji Maharaj, the author of Bhaktamal, also wrote the same about Tulsidas in his book, (1585)
"कलि कुटिल जीव निस्तार हित वाल्मीक तुलसी भयो"
(Bhaktamaal, 129 Chhappai, Page No. 131, Maluk Peeth edition)
Śrī Valmiki took incarnation as Srimad Tulsidas to cross the ocean of death for the creatures of this age.
श्रीवाल्मीकचि जाला श्रीतुलसीदास राम यश गाया । तारच प्रेमरसाची खाणी वाणी तशीच वश गाया ॥
(Ancient Maharastriyan Poet Moropant)
Shri Valmiki became Tulsidas only to sing the glory of Shri Ramchandraji; That's why the mine of such love-filled speech came under his control.
सर्वलोकोपकाराय प्रेरितो हरिणामुदा। वाल्मीकिस्तुलसोस्दासस्तद् रूपेण भविष्यति।।
(Sri Brihad Brahma Ramayana- 1.78)
Meaning: By the will of Sri Hari, Sri Vālmīki Mahārishi will appear as Śrī Tulsidās Ji for the benefit of all people.
भाषा काव्यं मानसाख्यं रामायरणमनुत्तमम् । करिष्यति जनानां यत्कलौ शीघ्र फलप्रदः ॥
(Sri Brihad Brahma Rāmāyaṇa 1.79)
Meaning: Shri Goswami Ji will make the best language poem famous by the name of Śrī Mānas Rāmāyaṇa which will be a quick fruit giver for all people in Kali-Yuga.
भव रोग हरी भक्तिः शक्ति यस्व शुभ प्रदा । ज्ञान वैराग्य सहितं कीलकं यस्य कीर्तिम् । तं मानसं राम रूपं रामायणमनुत्तमम् । प्रणमामि सदा भक्त्या शरणं च गतोस्म्यहं ॥ श्रीमदरामायणं दिव्यं मानसं भुक्ति मुक्तिदम् । यस्व श्रवण मात्रेण पापिनोऽपि दिवंगताः ॥
This Śri Mānas Rāmāyaṇa is the form of Śrī Rām himself which gives Knowledge, power, renunciation, fame and Bhakti of Śrī Hari. The one who even listen this glorious Śrīmad Mānas Rāmāyaṇa gets all enjoy in his life, will be free from sins and will get Moksha at last.