In this particular case it's because Bulma is using node-sass as a precompiler. I'd wager a guess this is true of most CSS packages that you're finding on NPM.

In this particular case it's because Bulma is using node-sass as a precompiler. I'd wager a guess this is true of most CSS packages that you're finding on NPM. Additionally, it's more pleasant to use than Bower (in my humble opinion).