One possibile factor is that ad blockers are reasonably easy to create. It’s possible that so many were created, they diluted the market, making it difficult for one to grab enough market share to crack the top 100 overall.
Another (perhaps obvious) factor could be the drop in attention, fewer bloggers writing about ad blockers. While iPhones are still selling like hotcakes, it could be that new buyers are not aware of ad blockers and the difference they make to the browsing experience.
Maybe as blockers aren't getting more and more popular.
I think both of Mark's reasons are having a small effect, but I don't think they're the primary cause here. It's much simpler than that.
The people who were interested in a content blocker downloaded it right away. The apps topped the charts for a bit while a large group of users downloaded them all at once. Then it stopped.
I don't think these will ever be something that the general public uses.