Oh, simple. SCSS is CSS compatible. So, if you are at a company, you can add Sass to your build-chain and coworkers who don't want to use it... don't have to!
Also, all of your old code works (as long as it was valid!)
Adoption was slower when you had to 'learn a new syntax'. SCSS made Sass just... CSS++
I use SCSS syntax now (it's the default), but I do really miss the +myMixin syntax.