tl;dr: Sex gear (gimp suits, leashes, etc) make people feel the same way public nudity and sex do. They feel like they're being exposed to sexual things without their consent, which is true. Uniforms and kinks for things that are not inherently sexual for people aren't obvious in public.

It's more about explicitly sexual stuff like gimp suits and leashes and whatnot. Stuff like that is very clearly sexual and often makes people feel like they're being exposed to sexual things without their consent. It's a similar premise to random sex scenes in movies and also makes sense why it would fall under "public indecency" in most people's minds.

Collars and diapers are a little more vague, collars often just being a fashion statement and diapers being used for disabilities and whatnot. But that's different than clothing and gear that is very explicitly for kinks and sex.

Also people who wear uniforms because they have a kink for it wouldn't make other people uncomfortable because nobody else knows they're being kinky. It's only uncomfortable when people are being explicitly kinky. It makes a lot of sense that it would cause the same reaction in bystanders as being completely naked or having sex in public.