Because it is the day of the Crucifixion.
Every Sunday is a “little Easter” in commemoration of Christ’s resurrection, and every Friday is a “little Lent” in commemoration of Christ’s sacrifice on the Cross.
I always thought it was kind of a neat pun, that Jesus gave up His flesh for us on a Friday, so we give up flesh (meat) on Fridays in remembrance of this.
There are a lot of people who are confused about this issue, and I remember becoming very confused about it when I was in RCIA.
I wrote to my Bishop about it at that time, in order to get clarification, and he wrote me back and said to abstain from meat, but if abstaining from meat isn’t a penance for you (because you are a vegetarian, for example) then you can do a Friday devotion such as the Litany of the Sacred Heart, or the Stations of the Cross.
But he said you must either abstain from meat (if you are a meat eater, normally) or do some kind of penitential devotion. He said it is incorrect that penance itself is “optional” - we do have to do
something - and the preference of the Church is that we normally would abstain from meat on Fridays.
When I discussed it with my priest, he said basically, use your head, and remember the law of charity - if someone goes to a great deal of effort to create a meal for you, and it contains meat, then, out of love for them, honour their effort and eat the meat, even though it’s Friday. But normally, we don’t eat meat on Fridays.