I had very little trouble finding all three ad variations of yellow poster wrappers in high grade 15-20 years ago. I think what we’re seeing here is a hobby that has lost a lot of steam from its heyday. It’s sad to admit that, but very few fresh collections of vintage material surface anymore, and the overall level of energy — from both sellers and buyers — is way off from peaks of the past.
Another factor - eBay has become a lot less useful in general due to years of sellers overpricing items, watching them sit, and everyone losing interest as a result.