That an honest question?
I guess they could have removed the disc drive (I don't think I mind it going, but right now other people want it, and want my stuff to be handed in on a disc) and put a desktop GPU there, provided they moved a few other bits around.
And, of course, 21.5" owners won't have to deal with a 5400rpm drive, and there'd still be room for the optional second drive. Perhaps they could have removed the option for the second drive (Bring on the Fusion Drive of course) and just have a bit more room for things.
The thinner design is totally unnecessary and the stand gets in the way making it totally useless as well -_-