Because it saves time and has been extensively tested across all browsers? And there's vastly more utility to Bootstrap than just a grid, forms and icons. It's

Because it saves time and has been extensively tested across all browsers? And there's vastly more utility to Bootstrap than just a grid, forms and icons. It's also not an all or nothing proposition, you can simply include the bits and pieces you want to keep the size down. e.g., @import '../../bower_components/bootstrap/less/grid';