I hate to sink to their level, but here is the explanation:

Adam and Eve were "perfect" humans with perfect bodies, perfect digestive systems, and perfect metabolisms. They were told by God to eat only fruits and vegetables because they were not sinners (Gen. 1:29).

Anyone who is not a sinner now should follow this advice because his perfect body can take this kind of diet.

As for everyone born after Adam and Eve sinned, they are all sinners and do not have the perfect bodies to be vegetarian. Therefore God said to the sinners:
Genesis 9:3
Every moving thing that liveth shall be meat for you; even as the green herb have I given you all things.

Thereafter, the faithful servants of God: Abraham, Isaac and Jacob ate meat (Genesis 27), Moses and the Israelites (Exodus 12 and 16), Aaron, the Levites and priests (Exodus 29). Eating meat became part of the Mosaic Law along with the initial ten commandments(Deuteronomy 12).

God promises meat to his people:

Isaiah 25:6
On this mountain the Lord Almighty will prepare a feast of rich food for all peoples, a banquet of aged wine- the best of meats and the finest of wines.

Christians are admonished to eat meat:
1 Corinthians 10: 25, 26
Eat anything sold in the meat market without raising questions of conscience, for, "The earth is the Lord's, and everything in it."

Jesus fed 5,000 people fish by a miracle:
Mark 6:39
Then Jesus directed them to have all the people sit down in groups on the green grass. So they sat down in groups of hundreds and fifties. Taking the five loaves and the two fish and looking up to heaven, he gave thanks and broke the loaves. Then he gave them to his disciples to set before the people. He also divided the two fish among them all. They all ate and were satisfied, and the disciples picked up twelve basketfuls of broken pieces of bread and fish. The number of the men who had eaten was five thousand.

Was Jesus trying to give 5,000 people cancer by feeding them meat instead of vegetables? Did he not have the power to feed them fruits and vegetables instead of fish?

Are you perfect or a sinner? Perfect people were told to be vegetarian. Sinners are admonished to eat meat.