That's because everything from 3E on is crap. Literally. Every time I go to Half Priced Books I see shitloads of that garbage, much of it in unused condition. Nobody wants it. I asked at one store about this and they said it never sells. They try transferring it to other stores, etc. Some of them won't even buy that crap at pennies on the book at this point.
On top of that, I think WOTC overprinted a lot of those editions. My suspicion is that 3E and beyond never "sold" as well as they claimed. They "sold" a shitload of stock to
stores, but I think much of that remained un-purchased by actual
customers. My theory is strongly bolstered by the tonnage of leftover, unused crap that turns up like sand at the beach every time I walk into a book store.
And then of course, there's the fact that a huge percentage of gamers simply went along with whatever crap was peddled to them by WOTC. I've seen the type so many times and have argued them to death on numerous forums, it's ridiculous.
First, you have the people who have to keep up with whatever latest shiny object is thrown in front of them, like rabid monkeys. They're never satisfied. They bought 3E and made house rules for it, only to "upgrade" to 3.5E and house rule that to death, only to "upgrade" to 4E and house rule that, only to...etc, etc, etc. It's like a mental illness, a compulsive buying disorder. If it ain't broke, don't fix it. 1E and 2E weren't broken, so no need to fix it. You can sell a new edition to these people easier than you can sell crack to a crack whore, and they're even willing to pay more. It's sad and pathetic to watch.

Once they get tired of their 3E stuff and 3.5E comes out, they try selling their 3E stuff to recoup some money so that they can shell it out all over again for a 3.5E collection. And again for the 4E collection. And so on and so forth. Now you know where all that lightly used store stock comes from.
Second, you have the idiots who think they're "saving the hobby" by supporting this crap and spending shitloads of money on it. I see this type often posting online about how sad they are after the fact for having shelled out so much money for yet another "reboot". Of course, that doesn't stop them from doing it all over again, next edition!

Oh, they want to "save the industry" for the next generation, or "save the hobby" for nostalgic reasons. I have a particularly deep disgust for this type, because they're insane. I liken them to the "environmentalists" who ride bicycles everywhere (while disobeying every damned traffic law known to man) because they're going to "save the planet" from fossil fuels. I wish it were legal to just run them over.
Third, you have the OCD collectors who believe that 3E and beyond actually has anything to do with real AD&D and actually has some collectible value. So they buy it all up again and again and again, edition by edition, and keep it stored safely and unused until they 1) run out of room, 2) run out of money, or 3) get slapped with an option by the wife - get rid of the clutter or sign the divorce papers. Now you know where all that virginal, unused stock comes from.
It's the same thing as comic books. The older, quality stuff is truly valuable because it was meaningful, enjoyable, high quality, and was not overprinted. Certain issues of the rarest comics have only 5-10 known copies in existence. The modern stuff (90's on) was all gimmicks, reboots, garbage quality, overprinted crap. Which is why it's in the quarter bin, or 5 for a dollar bin. You can buy 50 long boxes full of that crap for less than a single issue of a decent vintage comic from the old days.
Ah, humbug!