I think the reason is because designers are not into complex toolchains. Just the fact that you need to compile this CSS is a hurdle that makes this much less usable in a designers world.
I've often bemoaned the lack of variables in CSS for things like colors, especially because hex numbers are not very readable, but overall I'm not an advocate of these kinds of CSS pre-processors. It's not that I don't see the value, but I think they can easily become a crutch for developers who don't want to learn how to write clean CSS and utilize the cascade or refactor their HTML into a more sensible structure (hint: sometimes the most semantic class name is actually a presentational one, rather than duplicating a bunch of CSS, 'mixins' just serving to mask the smell).
The other thing is that cutting designers out of your project just because they can't or won't set up a Ruby environment is a pretty easy way to eliminate a lot of the best designers.
I've often bemoaned the lack of variables in CSS for things like colors, especially because hex numbers are not very readable, but overall I'm not an advocate of these kinds of CSS pre-processors. It's not that I don't see the value, but I think they can easily become a crutch for developers who don't want to learn how to write clean CSS and utilize the cascade or refactor their HTML into a more sensible structure (hint: sometimes the most semantic class name is actually a presentational one, rather than duplicating a bunch of CSS, 'mixins' just serving to mask the smell).
The other thing is that cutting designers out of your project just because they can't or won't set up a Ruby environment is a pretty easy way to eliminate a lot of the best designers.