Guy du Maupassant's "The Necklace" has been defined as one of the most "widely read and anthologized" short stories (eNotes "The Necklace" Study Guide-Introduction). Given the subjective nature of the question, numerous reasons can be given, and supported, regarding the popularity of the tale.
First, the ending of the story proves surprising to many readers. Readers, under most circumstances, like to have endings which are unexpected. Readers may become bored if they know exactly what will happen next. Second, readers may tend to have the mindset that bad things which happen to others is better than when it happens to them (a sort of "better you than me" mentality). Third, some readers like to see people punished for their misdoings. Given that Madame Loisel acted horribly toward her husband (ungrateful), some readers may feel as if the punishment was appropriate (she deserved it). Lastly, many people may like reading a text such as "The Necklace" because it emphasizes a moral lesson. Moral lessons are timeless, and the text's moral lesson makes the text timeless as well.