Bootstrap and Foundation are too heavy.
They're great for prototyping but every time you start a real, front-facing, project with them you have to overwrite lots of designer styles that do nothing to add to the functionality of the website. That's annoying!
Responsive has been built with that in mind. It has been specifically designed and coded to be as lightweight as possible to prevent the need to undo styles set by the framework itself and allow developers to write efficient code and speed up development time.