I have also noticed this as well. Think about Terry Gilliam of the Monty Python troupe-he's one of the six main guys, but you wouldn't see him as much.
Now this brings us to the Sesame Street characters. Consider the merchandise, and character groupings anywhere(websites, number of stuff, etc.) It was always a floparound and it went like:
*Big Bird, Cookie and Elmo
*Big Bird, Cookie, Elmo, Ernie, and Bert
*Big Bird, Cookie, Elmo, Ernie, Bert, and Oscar
*Big Bird, Cookie, Elmo, Bert, Grover, and Ernie
*Big Bird, Cookie, Elmo, Bert, Ernie, Oscar, Grover
*Big Bird, Cookie, Elmo, Bert, Zoe, Elmo, and Grover
*Big Bird, Cookie, Elmo, Bert, Zoe, Oscar, and Ernie
*Big Bird, Cookie, Elmo, Bert, Ernie, Oscar and Betty Lou
*Big Bird, Cookie, Grover, Bert, Oscar, Ernie, Ernie, and Prairie Dawn
*Big Bird, Cookie, Oscar and Ernie
*Big Bird, Cookie, Elmo and Oscar
*Big Bird, Cookie, Grover
*Big Bird, Cookie, Grover and Oscar
*Big Bird, Cookie, Zoe and Elmo
*Big Bird, Cookie, Zoe, Elmo and Oscar
*Big Bird, Cookie, Zoe, Elmo, Bert, Ernie
*Big Bird, Cookie, Zoe, Elmo, Oscar, Bert, Ernie, Grover
*Big Bird, Bert, Ernie, and Cookie
*Big Bird, Cookie, Bert, Ernie and Grover
*Big Bird, Cookie, Bert, Oscar and Ernie
*Big Bird, Cookie, Oscar, Bert, Ernie, Grover
*Big Bird, Cookie, Oscar, Bert, Ernie, Grover, and The Count
*Big Bird, Cookie, Elmo, Zoe, Grover, Oscar
*Big Bird, Cookie, Elmo, Zoe, Grover, Oscar, Bert, Ernie, The Count
*Big Bird, Cookie, Elmo, Zoe, Grover, Oscar, Bert, Ernie, The Count and Rosita
*Big Bird, Cookie, Elmo, Zoe, Oscar, Bert, Ernie, Baby Bear, Slimey
...you know what, I'll let you finish the rest because it's exhausting typing all this...but you get the general idea: notice that in several Monty Python sketches/routines(from Flying Circus, the movies, etc) the order was this: there were six guys: John Cleese, Eric Idle, Terry Jones, Michael Palin, Graham Chapman, and Terry Gilliam. nd it would go in a floparound-Palin, Jones, and Gilliam; Cleese, Palin, Idle, Jones, and Chapman; all six stars plus an extra or more(i.e: Carol Cleveland, Ian Davidson, Connie Booth)...yoy know what I mean.
Plus, there are like 14-15(or more)characters on Sesame Street, and a lot more in the past years, so it's hard to say who are really the main ones(some may say that it's Big Bird, Cookie, Elmo, Zoe, Oscar, Grover, Bert, and Ernie---especially in Japan, they're huge there) But I might as well as go with the eight(Big 8 as I always refer them to)
You have to consider that Oscar's muppeteer is ageing. Caroll Spinney has been with Sesame Street since the beginning and performing Big Bird takes alot out of him. In the 30th anniversary special I remember hearing that he could only hold his hand up for 15 minutes at a time and then he had to rest. So there could be less Oscar considering how Caroll is getting close to retirement age and probably will never retire.
I actually heard from somewhere(TP forum)that he(or she)was with Caroll at one point and said that he will not retire. Secret Squirrel said that whenever Spinney is busy, Matt Vogel subs for Big Bird AND Oscar. Plus, the two characters have been appearing quite a lot the past few years, now that they are going to focus on the core classic characters more often(and yes theyre both CLASSIC CHARACTERS) So there is no fear of any of the characters being used less(with the exception of Bert in Season 33---oh, and as for Season 34, things got pretty weird. Big Bird, Elmo, Cookie, Ernie, and Grover got their own spots; Rosita, Baby Bear and Telly pretty much the ones focused around a lot, and Bert, Oscar, The Count, and Snuffy---Oscar and Snuffy not such a big problem, Bert wasnt seen much and although they reran the number of the day clips, The Count hasnt made an appearance on the street. But not that they were decreasing the character usage, it's just that well...you remember when EMTV was selling Henson(company/assets), and Sesame Workshop bought the rights to all the characters from them...well after that SW realized that they no longer had a financial partner and so they cut down on the episodes(33 had 50, and 34 26) Okay to make things easier: Because of only 26 episodes, it was very hard to use characters evenly-and the choices being the newer faces(Elmo, Zoe, Rosita, Baby Bear, and Telly)and the classic ones(Bert, Ernie, Snuffy, Grover, Oscar, Cookie, Big Bird, The Count) nd in Season 35 Oscar gets his own spot on the show, Bert was seen a little more(I think), and same with Snuffy. And finally the Count made an appearance on the street...and the rest, you all know.
The bottom line: Classic characters will get special focus for the next years to come, and when any of their Muppeteers happen to retire, they will always get new ones. Big Bird, Cookie, Grover, Oscar, Ernie, Bert, The Count, and Snuffy WILL live on for future generations, so don't worry.
The times I can think of a complete ensemble: Macy's thanksgivingday parade(several years), MUppets Take Manhattan, The Muppet Movie finale, MFC, MC30, you do the rest)