Why is uBlock Origin not available on iOS devices?

Because Safari and its WebView process are still the only processes allowed to run JIT compilers.

Other Browsers on iOS are just unaccelerated Safari Webviews.

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.

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.

Look at how Adguard built their safari extension. It is basically an electron app that talks to the browser. Not really in the spirit of uBlock.