It's not hard to "receive permission" though, it's just showing HMRC that you've set it up properly and are following the rules wrt how salaries are shown.
But it doesn't work for min wage staff which then excludes huge swathes of companies like retail, hospitality, cleaning, care work...... And most public sector schemes can't use it at all.
And there's no point "advertising" it, it's up to employers if they do it, there is definitely more admin, which is why some don't. And risk around things like unpaid leave periods. And people do have to be able to opt not to use it, so you have to have the facility to run a non sal sac schemes alongside it. So, you can definitely see why smaller firms can't be bothered!
It doesn't save loads for the employee either, maybe £15pm. Obviously depending how much they contribute. The big win is for the employer, especially with NI now increased. And some employers gift it back to the employee in their scheme, which is nice (I worked for a couple that did this).