Incidentally, I honestly don't know why Americans prefer corn-fed meat. It seems fattier than grass-fed and doesn't taste 'right' to me, but I suppose that's simply because I grew up eating 'our' meat and got used to that taste. As you say, a preference thing.
We have a lot of corn. Cheap corn.
It's not good for the cows, and the resulting meat isn't great, but feeding corn to cows is a cheap and fast way to get cows to gain weight.