How Come?

To answer Bekkster's question, the following is speculative at most. Here's what I was able to find on the origins of "How come" in the sense of "why."
It seems to be a feature of Germanic languages:
German - Wie kommt es
Swedish - Hur kommer det sig
Dutch - Hoe komt het

Nor is it uncommon to find variants of this phrase in well-known English literature such as Shakespeare:

The Comedy of Errors, Act II, Scene 2
How comes it now, my husband, O, how comes it,
That thou art then estranged from thyself?

The Tragedy of Coriolanus, Act III, Scene 1
Sir, how comes't that you
Have holp to make this rescue?

It seems that "How come" is elliptical in that one must fill in the missing words to deduce what the original phrase was trying to say. One possibility would be:
How [did it] come [to be that], which is clearer than the Shakespearean version, which has already dropped the "to be" for the more favored abridged version.

Take Our Word For It offers a historical context:

That's exactly what how come? means. Surprisingly, it is American in origin, at least in that form. It dates from the middle of the 19th century, and its first recorded form is in Bartlett's Dictionary of American English: "How-come? rapidly pronounced huc-cum, in Virginia. Doubtless an English phrase, brought over by the original settlers, and propagated even among the negro slaves. The meaning is, How did what you tell me happen? How came it?"

Its predecessor in England was how comes it that...? That phrase was used by Shakespeare in 1607, in his Coriolanus: "How com’st that you haue holpe To make this rescue?" However, he was not the first to use it; we find it first recorded in 1548 by Hall in Chronicle: "How commeth this that there are so many Newe Testamentes abrode?"

That this phrase dates back to England would therefore leave no doubt in my mind of its distinctly Germanic origin.

Interesting how the pronunciation reverted back to "how come" instead of "huc-cum"...