The Creative Commons licenses aren't approved simply because they weren't designed to operate on code.1 Software has different components, especially source code and compiled code, which the Creative Commons licenses weren't exactly designed for (CC licenses are primarily designed for media).
You're probably wondering why the content of the OSI site is under CC. Simply put, it's the most appropriate license to place - a licensed designed for text. Yeah, Apache and others may work, but the best is CC, and it's used there. Just because the OSI maintains a list of software licenses doesn't mean that they must use one on their site.
The closest license to CC BY-SA is probably the GPL or LGPL.
1. An application was made for CC0 to be declared open source compliant, however, CC eventually withdrew their application due to the immense debate with certain issues of the license