Because it continues to be read! The reasons for it still being read are many--like most excellent novels, character, environmental, and cultural descriptions are rich in detail, and further the concepts established in the plot, and most importantly, move the plot of the story along. The many different threads within the plot intersect interestingly and weave a tale of much broader tapestry; not only is it about a couple committing adultery, but the needling impact on them and the larger Puritan culture. It resonates in today's culture, as it did when it was written, because it is both an historical (What happened in Puritan times) and psychological tale, containing passion, revenge, loss, grief, hope, and redemption, to name a few. The deepest reason it's a classic is it reflects these elements, these deep themes that keep expressing themselves in people, and in their literature, throughout time. A "Timeless Classic" .....but that phrase is redundant.