One reason is discoverability as they have a rather extensive searchable directory of open source software.

Another reason for quite a while was binary hosting, which github originally supported, then discontinued, but finally added again in July 2013. Additionally, the ability to use any open source license or combinations of licenses, as Google Code supported binary downloads during the time github didn't but only permitted one license per project and only from a subset of open source licenses (originally a small subset, later expanded). Google Code, of course, is sunsetting now. And github now supports multiple licenses as well as binary releases.