Because Safari and its WebView process are still the only processes allowed to run JIT compilers.<p>Other Browsers on iOS are just unaccelerated Safari Webviews.<p>In order to have a real Browser on iOS, with a real Web Extension API (manifest v2/v3) users would need to jailbreak their devices.<p>This has been going on for decades, and Apple pretty much gives zero effs about it.
I would guess it using a blocking mechanism that isn’t compatible with the plugin system that iOS safari supports. There are at least other ad blockers in the AppStore.