For me personally I'm not just thinking about myself and the clips I'd like to see. I believe kids would benefit from seeing the older clips.
I always feel it's because of the structure of the show now being the reason why we don't see too many old clips. Not so much due to kids not benifiting from them as it is for time constraints. Back when we had JTE, we did have the chance of seeing classic Ernie and Bert segments every so often. That's about it. It's because of the multiple segments in the show that stop them from showing many older skits. I think that now most of them are gone, we could see some older clips (and indeed we have seen a little of 1980's clips in recent episodes, including Sally Cruikshank animations) due to the fact there is more room for them to do so.
However, I don't see them doing anything from the 70's anytime soon. Part of me thinks its because they want to retire them.
Some older stuff I can't see them using. But most of it is older filmed footage of kids. The dated clothing and graininess of the film makes it look really old. And while I don't personally think kids would mind, someone at SW, maybe one of the educators, thinks it could be confusing to little kids. Or something. I'm just spitballin' here.