Because that is all that happened before He warned us of the coming judgement and the Lake of Fire. Jesus went to preach to those who had died (which was all that men were subject to before His Gospel was preached to them) and then, those who received that message by faith were moved into paradise, to await their bretheren, who will be allowed into Heaven. Those who wouldn't receive that message are the same un-believers in hell now that they were in their lives, as is everyone whose name isn't written into The Lamb's Book of Life. The details of these things are not given to us, but they are never swept under the rug, it is merely a matter of study and meditation. God didn't cover this earth with His Glory for nothing. It is available to anyone who will believe, but hidden from the 'wise' in their own eyes (read Logos_X).