I've been using Bootstrap more and more for mostly one reason: it's responsive grid. I'm sure there are alternatives, but it required very little effort to find Bootstrap and it does what I need it to do effectively. The other features are a bonus I can choose to use or to leave out.

Most of BS's javascript features need to be loaded separately and if you use LESS, you can scale back a huge chunk of the CSS if you don't plan to use certain CSS features.