I imagine it's because 9/10 people don't understand the fetish, including most site owners. It's about far more than 'wearing diapers' but people can't get past the idea of the eroticization of children.
Ccbill will not even approve a site anymore that has a stuffed animal in a picture.
thus:
Portraying a situation where there is unconditional love, affection and nurturing is wrong.
Humiliation and degradation seems to be ok though
