Title says it all: Why..

Because there's no point optimising the game for PC until it's finished...

Because this is how a lot of players will run it, or see it. I have a computer that can probably run it on high quality, but I'm getting it for xbox first.

The graphics development starts with xbox,because it's easier to develope for.
Todd as already said this.

The question should be, Why not?

Pretty much.

The XBox 360 is also a consistent platform, it doesn't have variable hardware. This not only makes it a convenient developement foundation (as greatcarbuncle points out) but also a convenient showcase since everyone knows what the system is capable of.

The Xbox360 version is the furthest along in development.

Well, if you have a good PC, now you know that you'll get even better graphics than what we just saw.

Game is not done yet and the graphics still look more or less great and its on an Xbox... Besides no gameplay on a PC version gives you something to look forward to in the graphical department =/