Just as "Christ" is a title, not a surname, so too with Darth. In the presence of a Sith Lord one uses the honorific Lord, as in Lord Vader; Lord Sidious; and (ahem) Lord Brawl. However when using the third person the honorific Darth is employed.

When Obi-Wan uses this term when fighting Vader in Episode IV:

VADER:
... Now, I am the master.

KENOBI
Only a master of evil, Darth.

... this is now generally taken as Obi-Wan using the term as a taunt to Vader, throwing his conversion to the Dark Side back at him.

Only George Lucas knows for sure.

While "Darth" may not be a surname in the star wars mythos, it IS most emphatically a surname. Most precisely it is a surname not too terribly uncommon in parts of both Sweden and New England as exhibited by several of my ancestors. It is also a very nice given name, and one that I would be able to wear a bit more proudly if anyone would believe it was my real name. Alas, so is life.

Etymologically speaking, it seems feasible that "Darth" is derived from the phrase "Dark Lord of the Sith". This derivation would also conveniently explain the use of "Lord" as an honorific address.

English has a long history of shortening cumbersome titles in a similar manner, such as Bosun, Ma'am / M'Lord, or Gunny. On the contrary, it seems infeasible that the prevalent common language a long time ago in a galaxy far, far away was based on English morphology.