The earth was still a primarily molten ball and hadn't cooled yet. Much of the gases that contribute to the effect are trapped in permafrost and ice at the poles. There's also, of course, the matter of all the previously trapped carbon being pulled out from underground that otherwise wouldn't ever have a chance to reenter the atmosphere. The atmosphere of the early and geologically overactive earth was also much less dense as much of the water and gases came after that period on comets.