Before answering you have to understand what offerwalls were good for; they were good for cheap installs. Traffic quality was attrocius and full of fraud, so you couldn't do pure ROAs campaigns for them. Knowing this the strategy for offerwalls was to buy enough installs to increase your downloading ranking to the point that the increase in organic installs from a top ranking position brought your campaign to be ROI positive.
For this to be even worthwhile or work you needed to hit top 10 download ranks. Even then it's broken into two tiers: Top 5 and top 10. Being top 5 meant that you were the first games people saw when they looked at download rankings. Being top 10 meant you were one scroll away.
So knowing all this, why are offerwalls no longer popular?
- Reduced visibility on the download rankings. It used to be that the 2nd thing any user saw on Google Play or the Appstore was the download rankings. Now it's at least a few swipes away and you have to actively look for the rankings, which drastically reduced the volume of organic traffic from rankings
- The download rankings are a relative ranking. Overtime as more and more smartphone adoption occured, it would cost more and more to hit the top 10 ranking. Both higher cost in terms of volumes and per install needed. There was a time around 2013 where it would cost between .10-.15 cents to buy an install and you needed about 25-30k installs a day to hit top 10. By 2016, it was closer to 2 dollars an install and roughly 80k installs. It just got prohibitively expensive from competition.
- Hyper casual games were the death knell to the strategy behind offerwalls. The advent of hypercasual games meant that getting a top 10 ranking was now a pointless exercise. They brought the daily volume to hit top 10 via offerwalls to a point where you don't even bother with it, think ballpark of 150k+ installs a day.
In short offer walls stopped being popular because they're no longer good at what they did. The UA landscape changed, advertisers and publishers simply moved away from it.