This makes it look like I'm asking for it... lmfao

Not personally into ABDL so can't speak exactly for it, but a big part of the appeal as it relates to other forms of ageplay (obviously between two consenting adults) is usually (though hardly always) some form of power exchange play where one or both parties take on a role of an age andor the general affects thereof usually for the sake of affirmation and support. Like, in some cases it can be sexual, in some cases it is not, in this case what seems to be appealing to her is the sense of giving up ones autonomy to trust her safety to her big, in this case the guy. The appeal behind that often the same as the appeal as being the sub in any BDSM play (ageplay falls within that spectrum after all, again between consenting adults), that is to say the sub enjoys giving up power and the dom enjoys enganging in and exacting power. In this case this takes the form of her giving up parts of her autnomy to take on a role where she can be taken care of, almost like an object with emotional needs.

ABDL (adult baby diaper lover) play is when that includes taking on kink gear like diapers and such and allowing oneself to be completely at the will of their big. More often than not this is for the sake of the emotional satifaction of giving up ones worries for a while and just being taken care of. Life is really stressful and just getting to be a baby and have your every need literally taken care of for you is very relaxing. This is generally what is known as "bright ageplay" which naturally falls into a spectrum where more intense themes, differing power dynamics (like little dom), and integrating other kink behavior can modify it to the intrests of the consenting adults involved.

Life is stressful, sometimes even traumatizing and it can act as a way to process these emotions in a safe enviroment with another consenting adult partner. Forget the world and just enjoy enjoy things with another consenting adult partner whom one has talked with and understands the bounderies and expectations and how to back out if its ever necisary

Apologies for being so pushy about the consenting adult thing, some people get *really* weird about this, for gods sake some of the people into this are survivors of sexual trauma working through their feelings and reclaiming hurt as something same and under their control as an adult.
Sources: member of a kink community, couple friends into ABDL, general discussion of the subject with friends including survivors of sexual trauma.

Didn't see that coming (100% sarcasm)