Why?
Because as much as I really wanted one and probably could use it, I am more than satisfied with my current Mac Pro and my MacBook Pro, both which do everything I throw at them save for the few tasks that do take quite long. Those few tasks that take forever I can wait a few hours longer, I'll survive.
The moment I saw it in the store, I was astonished to how small and beautiful it was. When I got home and looked at my receipt, I realized how much money I had just blown on a computer that I didn't really need yet.
Also if I'm really going to buy this, I would get a slightly higher configuration than available in-store. Sure it's fast now, but in 3-4 years, the current base configurations will likely be too slow. My current 4 year old Mac Pro is still at the top of its game.