The calculation to mine BTC blocks and process transactions, SHA-2 works very well multithreaded and parallelizes readily. This means hardware with a lot of cores like an AMD GPU can excel at it. Nvidia GPUs with less cores that each have a higher degree of sophistication such that they keep up in graphics processing lag behind AMD GPUS, because more cores and more threading helps more than the increased sophistication of the Nvidia cores. The i7 with its core count and being near the peak of sophistication doesn't stand a chance.
Or as someone one metaphored: NVidia cards are like having 4 MIT grads doing your all your math homework. Yeah, they can do pretty much anything you throw at them, but there's only 4 of them and they can only do so much in a given time. AMD/ATI cards are like having 1000 high school grad honors math students doing your homework. THey might not be able to do the most advanced trig or calculus problems, but they can do hundreds of times more basic functions in a given time period than the 4 MIT guys.
or something like that.