Grwat question, I was actually researching that this morning. What I understand is that Friday is the day Jesus made the ultimate sacrifice on the Cross. Traditionally meat was reserved only for festivities or times of celebration (think of the father killing tree fattened calf when the prodigal son returned) it wasn't thought appropriate to eat as if you're feasting on the day Jesus died so that's where it became a day of penance. St. Thomas Aquinas lays out a whole list of things people used to abstain from but which we're no longer obligated like dairy and eggs. Basically a full vegan diet. The green earthers would approve 😉