It would be easy to ask this question in hindsight. Why DIDN’T God warn Adam and Eve? I can’t conceivably answer that question, as His “thoughts are not our thoughts, neither are your ways My ways, declares the LORD,” (Isaiah 55:8). Any answer I can possibly give could not reasonably or realistically be acceptable as utter truth.
What I believe about the question, however, is this. First, God DID warn them about not eating the fruit. This alone is reason to say that Adam was in a state of willful rebellion. I don’t think it is necessarily fair to say I should have been warned when I was already told not to do something or else I would die. So, my contention is that by being told not to by God in chapter 2, they already HAD been warned.
Previous warnings notwithstanding, I do believe that there is another side to this issue. The other part to this equation is this: Love is a choice. Obedience, likewise, is a choice. Had God explicitly warned them, He would be effectively taking the choice out of the equation. Thus, I think that for God to have warned them further would be a removal of choice. Even with Adam and Eve making the wrong choice, God in His sovereignty already knew how the problem would play itself out and how He would reconcile His creation to Himself. Read Genesis 3:15-16 and consider that this is very much the first telling of the Gospel of Jesus Christ!