Sure, they've digitized plenty, but they haven't RESTORED all of them.
The original post did only say "digitize" but nothing of restoring or remastering.
Hmm, I wonder how often Sesame Workshop restores/remasters footage, or if there really is a need. Whenever something old is released on DVD/digital or whenever Sesame Workshop uploads an old clip online, the quality is really great (though I've heard some fans recently criticizing recent HD uploads of classic clips for interlacing, I'm not really sure if I see what they mean), whether it's something that fans have previously uploaded (and usually looking better) or a rare clip that we haven't seen or maybe even haven't heard of.
Another point is that all these episodes that have mysteriously surfaced, clearly from the vaults, are in such great quality - and clearly there weren't plans for these to be released to the public anytime soon. Though I believe I once read that Sesame Workshop used to make new copies of every episode when new video formats/new types of video were out before finally making digital copies and no longer having to convert the episodes. I wouldn't be surprised if they started making new copies when the early episodes were still in fairly new shape.