There are a lot of answers to this question. For me, it's a combination of a few things:
As a child, I was potty trained very late (roughly at 4.5 years, give or take), and I think it left me with memories and feelings about being diapered and being babied throughout my life.
I'm not going to go into detail, but I haven't had an easy life. Diapers, to me, take me back to a time where I didn't have to worry about a bunch of BS in my life. They take me back to a simpler, easier time. Along with that, I get the feeling of security, safety, warmth, and comfort with not only being diapered, but remembering those times.
Utilitarian. I, for the most part, am a very practical-minded individual. There's a certain level of practicality and use diapers have over bathrooms.
A kink. Yeah, I enjoy and love "playing" in diapers. There's just something different about it.
There are other things, but for the most part, they fall into those sorts of categories. Bottom line is, to me, there is nothing like diapers and being diapered.