This was brought up in the CSS WG's discussion of @when vs @if: w3c/csswg-drafts#112

@when was chosen over the far more common @if to avoid conflicts with Sass, a CSS preprocessor.
Web Platform technologies need to last for decades, and should not be designed around third party tools that just happen to be in use at the time, unless it is absolutely necessary because it breaks the Web (e.g. array.contains() that TC39 faced).