In my limited experience browsing the posts on various online ABDL forums and blogs, I've noticed many females in the community are puzzled by the amount of "sissies" and cross dressing that guys in this community tend to do. After thinking it over, I've come up with several reasons that could explain the prevalence of this phenomena. As a disclaimer, I am not a sissie, though I am a young male dl. Please feel free to call me out on any bias or misconceptions I may have.
First, and most obvious, is that in our culture (right or wrong) femininity and feminine beauty are associated with innocence. On the other hand, masculinity is associated with ruggedness, and definitely not cuteness. ABDL males may be uncomfortable reconciling the fact that they are men yet have a diaper fetish or enjoy pretending to be an infant. As a result, they take up the identity of a female, since the supposed attributes of this gender feel more appropriate when paired with their fetish.
Secondly, diapers are cushie (no pun intended). Or to put things bluntly, they can make guys' junk uncomfortable. I can speak form experience when I say they can make a full erection impossible (while having the diaper on). This can be sexually frustrating for DL guys. It could be fair to say we're jealous of female abdl's; I know I am. The mature female body is just molded more properly for diapers than the male is. This can lead to us having a strong desire to be female, since then we would actually be able to have sexual gratification while still wearing the diaper. Crossdressing/giving ourselves a feminine appearance is as close as we can get without going the whole nine yards with a sex change.
Last, let's face it, most straight abdl guys fantasize about females in diapers. If we aren't fortunate enough to see that in reality, we could always dress up as a chick in a diaper and use our imagination right? I feel like this is probably a motivation for many sissies.
Now I realize these reasons do not really cover gay or bi sissies, but they're motivation may be more obvious. I hope this may have provided some insight for any puzzled female abdl's. Feel free to leave comments, questions, and ideas of your own.