So many reasons, but the simplest answer is that we believe it's a better choice to handle the complexity of CSS in the Template, rather than the CSS level.
Even the best of CSS authors can suffer the Fragile Base Class Problem, and that is amplified across teams of differing skill level.
Not every project is a Javascript Single Page Application, and I am not convinced that it's a smart idea to require a build pipeline for every prototype, idea or product. Sharing the css utilities across projects and ideas promotes adoption of a component library.
Most UI libraries require you to include their CSS anyway, with the downside of adding that bloat for just their components. Using a functional CSS library allows you to power the Mandala components, as well as your own components with the same syntax.
If something doesn't exist in the Mandala Library currently, compose it and submit it!