Why is Dr. Jekyll and Mr Hyde so popular in modern times?

Strange Case of Dr Jekyll and Mr Hyde was initially sold as a paperback for one shilling in the UK and one dollar in the U.S. Within the next six months close to forty thousand copies were sold. By 1901 it was estimated to have sold over 250,000 copies. Many people think of it as a tale of duality and the struggle of doing what is right between the two sides - good and evil. Also the struggle of keeping two personalities in one body.