For the simple reason that I think it's likely to be full in the Prem and it has been shown that it won't be full if we're not in the Prem. As someone has already stated in this thread, it's tough to create a very good atmosphere in a non-full ground.
Additionally, really good atmospheres occur for many reasons, one of which is over-achievement. If a club is doing better than they are expected to do, or has the potential to do so (e.g. cup ties against higher opposition, most matches in the Prem etc), then that creates more excitement if and when things go well, manifested in a good atmosphere at the ground. Expectations at our club are currently so high that essentially no Championship game has this ingredient, which is one of many reasons why I think the atmosphere has not been good recently.
When you combine a somewhat empty ground with a slight apathy borne from the expectation that we should win, it's never going to be a raucous / intimidating / noisy / whatever-adjective-you-want ground.