Why was the book Frankenstein so popular?

Mary (then Mary Wolstonecraft Godwin) Shelly's Frankenstein was actually written on sort of a dare. While vacationing in Switzerland with Lord Byron, Mary and her future husband Percy Shelly and friends were housebound during inclement weather. To entertain themselves, they began talking, telling each other stories. The dare comes about when Lord Byron challenged the members of the group to to see who could create a supernatural tale. Mary Shelly had a nightmare as a result of the ghost stories and began to draft Frankenstein the next day.