There are a few theories about its origin:
* Contraction of "How did it come to be that": This is the most popular theory. Over time, the phrase might have been shortened to "How did it come" and then further contracted to "How come."
* Influence of German: Some scholars believe the phrase might have been influenced by the German "Wie kommt es," which means "How does it come" and is used in a similar way to "how come."
* American vernacular: "How come" became particularly popular in American English, perhaps as a reflection of a casual and informal style of speech.
Regardless of its exact origins, "how come" is now widely used in English. It's considered informal and casual, often used in conversation and writing for children.
It's important to note that "how come" is not considered grammatically correct in formal writing. In those situations, it's better to use more formal alternatives like "Why" or "How did that happen?"