Why did Adam & Eve partake of the tree of good & evil.?

Because it was given unto them and was a part of God's plan for them to become mortal and to fulfill the other commandment to multiply and replenish which they could not do until they had become mortal by eating the forbidden fruit. God knew that eventually they would partake, thus insuring the second commandment was fulfilled. He knew that they could not keep both commandments. He put them in a situation where they had to think and choose for themselves to obey or not. This act of exercising their agency, choosing for themselves, was in essence the beginning of knowledge and the loss of innocence. It was all part of the plan.