...because it is one. This is not by the original developers, and it shows. The character of the game is entirely different from the previous two. I seem to recall accidentally getting two species to wipe each other out in SC2. In SC3, you've got some "galactic United Nations" thing going where you can't even kill aliens without it *ending the game* (as opposed to, say, a strongly worded letter a real one might offer). Not to mention HAL and sock puppets.
This is a very enjoyable game. Its character designs are wonderfully over-the-top, and the music is great. My only objection is that Quit goes to the title screen -- clearly a console port, because I can't exit without Alt-Tab + End Task. :)
I love this game. The music is beautiful and calming and the gameplay is very challenging. Also, unlike Space Rangers 2, it works correctly on a netbook with Intel integrated graphics.