Regarding why this part of Barahin-e-Ahmadiyya was delayed, Hazrat Mirza Ghulam Ahmad (as) writes:

“The four parts of Barahin-e-Ahmadiyya which have been published consisted of such matters unless these are manifested, the arguments contained there in find their fulfilment. It was not to be that the work on the fifth part begins.” (Barahin-e-Ahmadiyya (Part V), preface p. 2, Ruhani Khaza’in, vol. 21, p. 3)

He further stated:

"Another reason for the delay was God wanted to divulge that his mission was in accordance with His will and all the revelations that contained in earlier parts are from Him and are not man-made. The great mission of the fifth part of the book was, that it constituted as a harbinger of earlier volumes. It was a great sign of God that He kept me alive till all the prophecies find their fulfilment. Now the time had come that the work on the fifth part should begin and at the time the work on it is beginning represented The Triumph of Truth and it is an acknowledgement of these blessings that I have named this part of the book as The Triumph of Truth and therefore some earlier pages 25 carry the title of The Triumph of Truth." (Barahin-e-Ahmadiyya, Part V, preface p. 7, Ruhani Khaza’in vol. 21, p. 9)