My guess is because you offer less product than GitHub, and charge more for it. Unless you make your value proposition clear, I don't see any reason for anybody to use your service.
A common theme seems to be that I'm not making the value proposition clear. Github actually charges $22/month for 20 repositories and 10 collaborators. Repodrop is $9/month for unlimited repos and unlimited collaborators. I will have to make this clearer.