So how come we say “how come?”, and is it current in all English speaking countries? And if it is current in your dialect, is it considered appropriate in all contexts, or only some?
Actually, the etymology can be easily imagined. “Come” in this phrase could be a past participle, so it might have been something like the following:
“How has it come that…”
But even if it is an infinitive, it could as well be,
“How did it come that…”
How far back is the phrase attested?