Because of the scarcity effect created, particularly on women, Shelley E. Kohan, a retail management professor at Syracuse University, told The New York Times, “Now this reusable cup becomes a fashion item accessory like a handbag or belt. Add a special color or a limited edition, and it’s a ‘fashion frenzy.'”

Source: https://www.buzzfeed.com/alanavalko/stanley-cup-trend-explained-history