When WebKit Content Blocker extension was introduced in 2015, a slew of indie developers have flood the App Store (at least on iOS side). Most of these apps were abandoned shortly thereafter, even successful ones like Purify.
1Blocker is one of few that have stuck around, with new update every few months and taking advantage of the latest iOS features. Let’s face it. Advertisers are always trying to circumvent ad blockers and sites occasionally break, so continued developer support is critical. The developer responds to issues quickly and many are fixed quickly.
Also important is sustainable business model. I am not sure if 1Blocker makes meaningful money, but it is a paid app (freemium on iOS side) and it does not monetize from advertisers.
1Blocker is also one of few that support macOS as well, via WebKit Content Blocker and iCloud for syncing rules.