Green teas are harvested from the plant and fixed immediately. This means there is minimal oxidation of the leaf.
Think of it in terms of color and how oxidation would affect the cellular structure of that leaf. Oxidation is allowing air to invade the burst cellular walls of the tea leaf. Because no (minimal) oxidation is allowed to happen, the chlorophyll is undisturbed and so the final product remains green.
To stop oxidation (or denature), leaves are baked to roughly 150° F.