Why? I don't see it in God's word. It is a tradition but I really can't see it as biblically supported.
Nobody says it’s mentioned explicitly in the Bible.
Fasting is practiced in the OT and the NT. Abstinence from meat on Friday is a form of fasting.
Where in the Bible does it say that we are allowed to do only what is explicitly mentioned in the Bible? Hint: nowhere.
I think the key part in that response is that it is a sin for CATHOLICS to eat meat on Friday without an adequate reason. They can make the rules for their own members but they cannot insist their rules apply to ALL Christians. There seems to be a lot of confusion among Catholics, though, about meatless fasts on Friday throughout the whole year or just during Lent. Ask 10 Catholics that question and you just may get 10 different answers.