As more of an Adult Baby than a Diaper Lover, I can tell you that in the past, the question was not simply "why diapers?", but as LittleMoosey has said, it was "why anything?". Why act like a baby? Why wear onesies and baby clothes? Why a pacifier? Why drink out of a baby bottle? Why want a Mommy? The list goes on.
I use to have these questions from time to time, when I was still exploring and discovering. These questions came from a place of embarrassment and to a certain extent, shame. As a man, society often dictates that we need to be hard, we need to be rugged, we need to be detached emotionally, so wanting to be a baby again conflicted with that, being a baby makes one cute, vulnerable and emotional. Also, being a child raised by a single parent, my Mother raised me to be mature from a very young age, so certain childish things were frowned upon. So, you can imagine how hard it was to reconcile that I was to be a mature young man even though, I wanted to be put back in diapers, act like and be treated like a baby, pretty much the polar opposite of being a fine young man. Take in those two messages together and who wouldn't be asking, why?
Like you, however, I have since stopped asking these questions. Why? Because I have found my answer; my AB side is a part of who I am, it makes me happy and it's great therapy. So, why diapers? Why be a baby? Why not? The pros so outweigh the cons, in fact, the cons are pretty much nonexistent at this point and I have reached a position of full acceptance, so there is no longer a need to question this anymore