Question: are there any mature CSS component frameworks out there aside from Bootstrap and Bulma? And why aren't there more?
There are lots, it's just that many mostly advertise themselves as component libraries for particular frameworks. Oftentimes you'll find that the common core is just plain ol' CSS with a little bit of JS in the background.
- I want something lightweight and not bother to deal with compiling/preprocessing (like Milligram).
- I want a MVP without classes (like Picnic.css)
Otherwise I pay a premium theme or create one myself.