I think the reason is pretty simple. This game is just not for everyone. Most players who stick around have played this before, some at release myself included. For new players it has a very steep learning curve, it's nothing like other RTS out there, which is a good thing. SC is a very good RTS probably one the best, but it's not new player friendly. So i dont think there is anything to worry about. People who like it will stay around.
As the title reads. Specifically when creating games.