Apple made the decision when it released OS X El Capitan. To date Safari supports your browser extensions, both those published in the Mac App Store, and the extensions that developers distribute through their website. For security measures and integrity of the Mac operating system, the company announced that only extensions downloaded through the Mac App Store will be operational.
We will receive shortly Safari 13, surely with the output of macOS Catalina, and we will find that the extension downloaded outside the Mac App Store does not work, but the same extension does if we install it from the Mac App Store.
Apple ensures the integrity of this extension with this measure and we avoid the inclusion of malware in our system. The problem is found when the extension that comes "like a glove" is not available, since the developer does not have it adapted. In addition, unless we contact the developers, we will not even know if you are working on the version for Safari 13, within the Mac App Store.
Some developers may not be interested in adaptation extensions. Also, it's not just about moving an extension from one site to another. Change the language and maintenance criteria, update, in this case, by those set by Apple. On the other hand, publishing in the Mac App Store requires being registered in the Developer Program. This forces you to make a profit from the application, at least to cover the cost of subscribing to the program, which is $ 99 per year.
Also, on a system as optimized as macOS, the need for browser extensions is minimal. Therefore, predictably do not have much demand, which represents another additional barrier for the developer. Today the extensions with the highest demand have to do with blockers. Then we find extensions like pocket, for subsequent consultation of the webs or CamelCamelCamel, to follow Amazon prices, which today is not supported in the Mac App Store.