They could still choose a specific release date after pre-orders go live though. That is why companies pick the 31st because it allows them a bunch of time to delay if needed. They can also delay to the next year and at worst they have to offer a refund.
There is little reason to speculate on why pre-orders are not live yet because it could be anything.
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Which is normal for a "reveal". Even when at a full fledged blizzcon we don't get much more then they showed.