Safari is migrating to a new system of extensions that will make it much easier to port from Chrome. However, I understand it still requires Xcode (which non-Mac folks can't run) and a developer license (which not everyone wants to pay for). I hope to bring my Chrome extension to Safari, but honestly it's not a priority because most people who install extensions are not running Safari (when you consider that most people are not on Mac, and a large chunk of folks on desktop Safari are there because it's the default — and therefore would not likely install extensions).