I was fairly "loyal" to 960gs, but when Bootstrapped was released, I switched right away. Bootstrap was responsive, which was a huge selling point, and contained a few components and javascript features, which were nice to have all bundled up in one package.
Bootstrap's documentation was another big advantage. It's a larger library than pervious grid systems but it's modular structure and excellent documentation lets you get started easily, while adding in deeper features with little friction or confusion. I also think Bootstrap's roadmap looks positive, as it is placing a larger focus on making quality mobile experiences as simple as possible.
I think the excellent documentation is one of the biggest things that's contributed to the success of bootstrap. As someone who started using bootstrap after literally having no experience with CSS/HTML/JS, the absolutely fantastic documentation means I have used bootstrap over everything else. It's very comprehensive, very approachable, and excellently designed :).
Bootstrap's documentation was another big advantage. It's a larger library than pervious grid systems but it's modular structure and excellent documentation lets you get started easily, while adding in deeper features with little friction or confusion. I also think Bootstrap's roadmap looks positive, as it is placing a larger focus on making quality mobile experiences as simple as possible.