It seems that a large motivation to not use JS frameworks is their large size. Of course, the same can be said for CSS - the frameworks are quite large. Unfortunately, since most developers don't interact with CSS as much, people typically ignore the fact that Bootstrap and Foundation are 130kb and 150kb respectively - at the same time they complain about a 24kb JS library's size.

This inspired me to create Min, a 995 byte CSS framework: http://minfwk.com