Good question. Mostly that a "Mac mini" form factor allows for a larger hard drive, and that hooking a 30" monitor up to the new Macs either requires a crazy DisplayPort <-> Dual-DVI adapter, or maybe you can get the new Dell 30" that has (non-mini) DisplayPort and good luck finding the magic cable.

Also, the laptops cost extra money, since you're buying another display, and a battery, and so forth, that you don't really need. (And, I really don't. I have a MacBook Air that serves my laptop needs quite well.)