Why does Macbeth still appeal to us today in regards to theme guilt and conscience?
I think there are two answers to this. First, it is just Macbeth, rather Shakespeare that provides the allusion (the continuous appeal and use) for our current times. Shakespeare created amazing works of literature that spoke to nearly every human topic imaginable, and really began a literary tradition, which is why we so often see works emulate his originals or refer to them. Shakespeare is timeless.
Second, Macbeth as a character and as a play, is Shakespeare's most conflicted tragic character. It is the greed that consumed him and his wife that drove him to become a murdering machine, and a mad man. Greed and conscience are themes present in any society. Especially with the current economic situation and how it started, people were greedy. People wanted more than they had, even if it already was a lot. For the common folk (non-royalty in Shakespeare's time) and middle class people today, Macbeth is a model on karma in a way. Those who are greed and think only of themselves will end up paying for their ways through their own destruction. Macbeth, like Shakespeare, is also timeless. It is sad to say, but I do not think greed will ever leave our society, so there will always be a need for a story like that of Macbeth.