Why is the nexus 7 a quad core but the nexus 10 a dual core?
Any insight into why Google did that?
Why make a tablet with a superior form factor (10"screen) and then cripple it with a processor inferior to the 7" mini tab?
I'd be hesitant to call 10" tablets inherently
superior ? there are advantages and disadvantages to all screen sizes and aspect ratios. In the same way, I don't think the lack of a quad-core processor will "cripple" the Nexus 10.
Starting with 4.0, Android's UI has been increasingly hardware-accelerated, meaning the work is done with the GPU rather than the CPU. The only situation in which a dual-core CPU represents a "crippling" disadvantage in comparison to a quad-core CPU is in highly CPU-intensive gaming and number-crunching applications. The UI, as well as demanding graphics, etc. are all handled by the GPU. Whether the GPU is adequate or not is another question entirely (one which I personally have yet to even think about), but I don't think that you'll run into many issues with the Nexus 10 as far as processing power. Android's multi-tasking and multi-threading methods are resilient and relatively efficient.
Also, many applications aren't developed to make use of more than two physical processing cores.