The Workshop has kept a relatively good record of all of their material, thousands of hours of videotapes and films, and everything in between. But there is a reason none of these rare clips show up so often. It's because they didn't test well with kids, and couldn't hold up to the other more popular skits and songs.
Sure, it would be great to see some of these old things again, but they just aren't relevant to the majority of today's children, and that's who the Workshop aims for. Besides, they have bigger fish to fry, such as the continued production of new, more accessible Sesame Street shows for kids all over, plus various other educational programs and initiatives.
In short, none of these clips are really lost. They just aren't going to be seen anytime soon officially, due to their ephemeral nature. They are relevant to nobody but the hardcore fans.