Lehi moves from the general to the specific. From Lehi's embedding the principle of opposition (and action) into the very creation of the earth, he now moves to the implementation of that principle in the circumstances of our lives on earth. That must begin in the Garden, where the conditions of opposition and action were set up.
Lehi establishes the two trees as opposites in the Garden, thus making the Garden the microcosm of the entire creation. Where Lehi has argued that all creation has required this opposition, Lehi focuses that entire argument on one event in the Garden of Eden, using the two trees as the symbols of all such choices.
It is interesting that in normal parallel terms, Lehi establishes two trees and then two attributes. If we were to assume a normal parallel structure, we would have the following:
The parallelling of the themes would lead us to assign bitter and sweet to the trees, but where we might naturally assume that it was the forbidden fruit that was bitter and the tree of life that was sweet, the direct parallel would belie that, and have the forbidden fruit be the sweet oNephi
Why such an alteration of our expectation? Precisely because it was the eating of the forbidden fruit that generated the conditions of opposition, and created Lehi's bitter/sweet dichotomy. It is precisely that forbidden tree that is the focus of the events of the Garden, and Lehi's argument that it was critically important for Adam and Eve to eat of it.