Why didn't any of the Classic Muppets(other than Kermit)ever appear on Sesame Street?

[Muppetage]

My guess is that they're too hardcore and not very appropriate for Sesame Street.
Miss Piggy often hits on good looking men.You can't do that on Sesame Street.Plus her "HYAHH" probabley wouldn't gel on Sesame Street either.
Nor would Gonzo's wild stunts.
Can you imagine Animal on Sesame Street?

 

[TheRealFraggle]

Part of the reason might be that most of the "classic" Muppets (i.e. Fozzie, Miss Piggy, Gonzo, etc.) didn't even exist at the time Sesame Street debuted, and by the time The Muppet Show came around and those characters became popular, Sesame Street already had its own well-defined cast of Muppet characters, so seeing characters from a different show appear would have been strange. There were a few crossovers though, mainly A Muppet Family Christmas, and of course some of the Street puppets were in the first three Muppet films.

For what it's worth though Rowlf appeared in one of the "baker films" very early on though, so maybe that counts .