Without reading the docs in depth, there's a difference of about 142KB of render-blocking CSS. I'm sure the API and functionality depth is very different too.
People using Bootstrap end up serving a lot of CSS that isn't even used in the end. Using a minimal framework like that one allows you to have a solid skeleton without any extra features on which you can build.
I'm trying to come up with a situation where loading a 199kb css file would make a difference. Maybe websites that target an audience that is expected to have a slow connection (banks etc.) but this kind of rules them out by not supporting IE.