I went through several projects where team leads laughed at the idea of Bootstrap in their projects. They treated Bootstrap as a bulky, unnecessary mess of a framework that only inexperienced code monkeys used in spite of it being small and modular.
The dumbest one of all was a team lead who turned out to have little to no real world experience in decision making. He had some help creating an internal CSS framework. It was responsive, modular, mobile first, and had lots of builtin classes for font/element sizes, colors, behavior, etc. Sound familiar? His column-based framework was a terrible drop in for Bootstrap. Even the class attributes were blatant rip-offs, but the lack of robustness meant lots of inline CSS to fix elements.
Prior to Bootstrap, the concept of a purely CSS framework was relatively foreign. Aside from jQuery UI (which of course required jQuery and had limitations on styling) the closest thing we had in the 2009-2011 period was reset.css and 960.gs. Even rudimentary things like rounded buttons were still impacted by the lack of uniform border-radius support plus an IE6/IE7 marketshare greater than 30%.
When you just want to move a button ... "oh they're using bootstrap" often saved a ton of time, for things that really shouldn't take time.
I think the bar was low for good design on the web, and they did a really good of making it out of the box seem like "good design".
I remember the first time I saw an open source thing was bootstrap based, without knowing what bootstrap was, I immediately asked "wait, what did you use to make it look that good, and how can I use it?" I could guess it was using some third party tool, it was just so much better than typical.