Apple made a tool to convert extensions a couple years ago that makes is pretty trivial to get extensions on Safari that didn’t used to support Safari.
I wouldn’t go as far as to say it’s a “set it and forget it” situation, but it requires minimal work in most cases. Hour or two tops.
I’ve helped out and converted a few for some projects and converted a few for private use via source code found on GitHub.
At this stage I’d say it’s mostly a lack of interest by extension makers if they don’t offer support for Safari.