Definition of how come? in US English:

how come?

phrase

informal
  • Said when asking how or why something happened or is the case.

    ‘how come you never married, Jimmy?’
    • ‘I told him, ‘If we are not China and we are not Taiwan, then how come?’’
    • ‘She frowned at me, looking disappointed, and he raised a curious eyebrow, asking, silently, ‘how come?’’
    • ‘He said nonsensical things like, ‘You're so many colors all over, how come?’’
    • ‘He rubbed his palms together ‘Could you explain to us how come?’’
    • ‘He smiled faintly at Michael, ‘I don't mean to sound pressuring or anything, but how come?’’
    • ‘Long-lost customers show up saying ‘Wow, heard you were closing, how come?’’