As I said here (https://news.ycombinator.com/item?id=7032169) it's usually because of front-end performance. Users are very concerned with performance, and loading a 150kb library doesn't help.
The extra kilobytes translate into seconds at an alarming rate if you're not on Google Fiber, and I've found that in rural NH Bootstrap usually takes 3 seconds to load vs. Min takes 300ms to load. Min (http://minfwk.com) is a 995 byte CSS framework I made to deal with the slow loading times of other CSS frameworks.
The extra kilobytes translate into seconds at an alarming rate if you're not on Google Fiber, and I've found that in rural NH Bootstrap usually takes 3 seconds to load vs. Min takes 300ms to load. Min (http://minfwk.com) is a 995 byte CSS framework I made to deal with the slow loading times of other CSS frameworks.