why did god initially forbid eating a fruit not porg, shellfish and beef?

This had NOTHING to do with dietary habits, it had everything to do with free will, and obedience. Adam and Eve, had the whole world at their fingertips with Blessings untold, their ONLY rule or restriction was, do NOT eat the fruit from that one particular tree . . . was that so hard? I can see how it would be kinda easy to be tempted to eat of the fruit, BUT, God had been there and walked and talked with Adam, spent time with him and taught him. Adam SHOULD HAVE been faithful to God, Who had proven Hiimself to Adam, but Adam choose to listen to and obey someone or something that had NOT proven itself in any way. If Adam and Eve hadn't screwed up, you and I would have I'm quite sure.

People did not eat meat at the time. People started eating meat after Cain killed Abel and God allowed humans to eat meat only after Noah's flood. Before that they where allowed to eat only fruits, vegetables and herbs. That is because the climate allowed people to live just with vegetables. I am unsure what was the status quo on animal milk before the flood according to God though.

He forbid eating the fruit of one tree

'Fruit' is symbolic

like the rest of the bible

it's not supposed to be taken literally.

When people do, then they go to war and reject scientific discoveries

It didnt, the abrahamic god is a myth.