Why are they called "urban" legends?
Answer: Fair question, given that urban legends don't always take place in urban settings, nor are they exclusively told in big cities. It's helpful to note that urban legends are also called contemporary legends, a term that's more literally accurate and preferred by some folklorists.
Various other names have been suggested and applied, but urban legend stuck, presumably because it distinguishes in a picturesque way between the kinds of legends told in bygone days, primarily by rural folks in rural settings, and those we tell now (although, in truth, many "contemporary" legends borrow motifs and storylines from traditional folktales of the past).
It's also evocative of a recurrent theme in contemporary legends ? what one might call the "dark side" of modern life ? namely, that new isn't necessarily better, and that many of the changes wrought by modernity, exemplified first and foremost by the impersonal hustle and bustle of big-city life, have come at the cost of the safety, sanity and sense of community our forebears enjoyed.