In Safari 14, Apple made it much easier to port extensions from Google Chrome to Safari.
I believe some improvements were even made in Safari 13. I would much rather use uBlock Origin compared to AdGuard. What’s preventing Raymond Gorhill from porting his extension to Safari now that it’s much easier compared to a few years ago? After all, Safari is generally much more popular than its competition.
I don’t have a direct answer to your question but I’ll offer a couple of observations:
Just because it’s (supposedly) easier to port extensions to Safari doesn’t mean that the App Store will accept them, and
It seems that the “improvements” that were made to enable Safari extensions has been met with a giant collective yawn from the development community. Dunno why.
The real reason is documented here. Basically the WebExtensions API that Apple implemented in Safari 14 fails to support the functionality that is needed by uBlock Origin.
Evidently it contains a work-around to the missing functionality or, for some reason, doesn’t require that functionality. Your guess is as good as mine.
If you want to prompt a developer to port or create an extension for Safari, perhaps ask the developer? Odds are the developer is not reading this forum looking for new customers.
The answer is as simple as Adguard not using the unsupported APIs and being willing to make the performance and utility tradeoffs required by the workarounds. @jec0047 's link has the latest official response from the developer, commenting on the APIs, and another statement from the developer on why he won’t be changing the plugin to accommodate (essentially, AdGuard is a business that can justify it, uBlock Origin is not.) He recommends Adguard for Safari users in the project’s Github readme.