Because they upload the classic clips on YT specifically for us, the show itself is specifically for kids, and the show - like many shows on PBS - has to follow certain guidelines for the sake of the kids: they have to avoid outdated material, because they either won't make sense to the kids watching, or the educational content doesn't meet the requirements of today's preschool kids. Remember, back when SST started, their goal was mainly to teach basic educational concepts like letters, numbers, shapes, and such: we've come a long way since then, and SST's done their job so well they have to step the educational content up a number of notches because letters and numbers are small potatoes for kids today. SST's a lot more "advanced" in its educational value today compared to then.