This wasn't an easy decision to make, but here were some of the deciding factors:
I spend a lot of my time debugging video drivers, and it makes me very grumpy. You guys seem to think that you can plug anything with copper and wires into your PCIe socket and call it a "video card". By restricting version 10 to iPad only, we cut down development time by targeting only one GPU.
We love everything Steve Jobs does and kiss the ground he walks on. Steve says the pad is magical.
The biggest single feature request we get for X-Plane is "can I use X-Plane while operating a motor vehicle." X-Plane 9's requirement of 1 GB of RAM, a modern CPU, etc. means that most X-Plane 9 machines are not particularly portable. By making X-Plane 10 iPad only, we can provide X-Plane in a portable format that's more compatible with flying a flight simulator while driving.