how come follows a statement and asks the question "why" or "in what way," as in You're not going? How come? The related phrase how so? functions the same way, as in You say she's changed her mindhow so? How come is short for how did it come about that and dates from the mid-1800s; how so, short for how is it so or how is it that , dates from about 1300.
Dictionary.com presents 366 FAQs, incorporating some of the frequently asked questions from the past with newer queries.