It's just that stuff like The Dark Knight Returns was always going to increase in value, regardless of Dawn of Justice's hype, and also regardless of BvS' subsequent 'downturn' because of the Theatrical Cut.
On the flip side, Death of Superman saw a bump, and a subsequent fall. I personally love the series, but some don't, and it was printed in abundance.
Movie hype will always get a rise and fall because of the 'sugar crash'. People aren't talking about it anymore. People aren't speculating about it anymore. There are always outliers, but this is my detective observation lol.