There's nothing forcing you. The code is right there in the repo. You're welcome to download it via Github's download button, cloning via git, or NPM. NPM is a very helpful option because it's the fastest and easiest way to get it into my project and not have to copy and paste stuff around either from my OS UI or command line. Just `npm install` then possibly writing a line in a gulpfile to copy the css where I need it if using unaltered.
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).