I'm not exactly sure why some people hate on the ABDL community but I guess it's just because they don't understand the ABDL community and it probably seems weird to those people that some adults like to be babies and wear diapers when they have no medical need for them.
I'm not an ABDL but I don't hate on the community either.
Actually I find it an interesting community and find it cute that some people like to regress into being a baby or toddler.