The answer
In the beginning God created man vegetarian. It is only after the flood that God allowed man to eat animals' meat.In the Holy Book of Ezekiel, we read God's instructions to Ezekiel on what to eat and how much he can eat and drink during the fast God had commanded him to observe. All the ingredients were rationed vegetarian ingredients, (wheat, barley, beans, lentils, millet, and spelt); and drinking was also rationed (Ezek 4:9-12).
Fasting is a time of repentance, a time when we ask God to cleanse and purify us and make us return to the original image in which He created us. Thus to abstain from all animal products is a way to go back to what God had intended for us before sin came into the world.
Originally all the fasts were abstinence from all animal products; but to make it easier on the people, the Church decided to observe this restriction only on the fasts of Wednesdays and Fridays, in Nineveh's fast, Paramouns, and in the Holy Great Fast. During all the other fasts, seafood is allowed.
Answered by His Eminence Metropolitan Youssef (opens in a new tab)