To me it seems very simple: level of effort. Uploading your hack/mod/whatever to rapidshare takes about one minute, or less. On the other hand, if you want to learn about git and github, you have to spend plenty of time on that.
Zero knowledge of git was necessary. Oh and also if you want to edit that README.md? You can do it from inside Github too, still zero need to know git.
(Plus I suspect that if a lot of people started hosting multi-megabyte binaries on github, their policies would change pretty quickly)
But yes, the most simplest site is the best for the most simplest people. However we're talking about people who spent a lot of time into creating their mod/whatever here. They can spend a minute or two more to figure out distribution.