If the book claims prose is difficult to memorize and unsuitable for recitations like Parayans, why is this preface itself written in prose?

That's an insightful question. The author of the preface, writing in Chapter 0, is expressing a specific viewpoint about the ideal format for texts intended for devotional recitation. He states that for 'study, recitation, Parayan, or Saptah, detailed poetic biographies are most suitable' because prose is hard to memorize. The preface itself, however, serves a different function. It is an introduction meant to explain the principles and context of the main work. It is commentary and not the object of the recitation itself. Therefore, it is written in prose to clearly articulate these foundational ideas before the main, likely poetic, biography begins.