I'm not entirely sure I accept the assumption of your question: Is it really any more difficult to imagine life after death than life before death? Many philosophers have argued that it is difficult to imagine being dead because the act of imagination requires that one be alive. In other words, any attempt to imagine being dead is thereby a failure, some have argued. In imagining oneself dead, one must presuppose that there is a consciousness (a living one, presumably), so one cannot coherently imagine being dead — at least if that means imagining oneself being dead. Now if that's correct, then one similarly could not imagine the past before one existed. After all, in attempting to imagine the past, that would require you to be conscious and to be alive, etc. Of course, one might take this reasoning to show that it's not any harder to imagine life after death: Since we can imagine what existed before our birth, we can equally well imagine life after death.
So I'm not entirely convinced of the assumption. But supposing the assumption is correct, why might it be harder to imagine life after death than whatever existed before our births? I can think of two reasons.
The first is that the past is determinate, whereas the future is either indeterminate (what the future will be like has not been settled) or we don't what the future will be like. It's generally much easier to imagine what is determinate than what is indeterminate. It's easier for a parent to 'imagine' the child she already has than it is to easy to imagine the child he will have. Similarly, it's easier to conceptualize the past since we have a much better idea of what it's like: Imagining 1916, say, seems easier than imagining 2116.
Second, imagining life after death may be harder because it's harder to imagine the world after I'm able to exert causal power over it than it is to imagine the world prior to my being able to exert causal power. I have no ability to change the world prior to my existence. While alive, I have the ability to change the world. Once dead, I no longer have that ability. Perhaps there's something difficult about imagining a future state of the world that I cannot affect. After all, throughout my life, I have been able to affect the future. Once dead, despite that being the future, I can't effect it. And perhaps it's tough to imagine a future state of the world without also imagining that one can causally engage it.
(Incidentally, you may the Lucretian asymmetry problem of interest: http://plato.stanford.edu/entries/death/#3)