I had a friend who told me that the a decree was issued by the church to abstain from meat on Friday in an effort to help the fishing market, but I'm not sure how true that is, I've never researched it myself.
I've heard that, too, with a slight variation for post-Reformation England: It was to support the fishing industry, because it was from the fishing industry that the navy derived extra sailors needed in time of war.
But that certainly didn't apply to the Mediterranean world of the early first millenium. Perhaps it had something to do with the value of cattle and sheep for other things besides their meat (dairy products, wool), but I don't know where hogs would fit into that scheme. (Muslims also don't eat pork.)
I'm sure the idea of fasting on Friday because it's the day of the Crucifixion must factor into the justification somewhere, but why fish should be considered permissable on a fast day when other forms of animal protein aren't brings us full circle back to the question again, I guess.
The meat/milk prohibition is a puzzle, too, especially when you think of it in terms of not boiling a calf or kid in its mother's milk, but maybe that was originally some twist on not butchering a cow or sheep or goat while it was still good for dairy products. Today farmers send cows to slaughter when they're no longer good milk producers.