Since sex is a topic that people find interesting, it stands to reason that a character whose primary established trait is the desire and ability to have sex with multiple people will be interesting to the audience. This principle also works when applied to other elements, such as plots, settings, or even the show as a whole. Bisexuals, when they appear in fiction at all, tend to get shoehorned into this trope because writers still aren't sure how to use them. The same is true for homosexuals, although some progress has been made in portraying them with more diverse personalities.