Sesame vs Muppets

Alright, will someone explain to me how the Muppet Show can get away with the outfits that were worn in the 60s (?) 70s but Katy Perry can't wear her dress with Elmo? I know this happened a while ago but it really has been bugging me... You'd think, knowing their parents allowed them to watch what they watched, that they would be *less* stuffy rather than more so on the subject for their own children... I don't know. I'm just curious.