Originally posted by: Yoda Is Your Father
The answer is because when ANH was made, Lucas didn't have the idea that vader was really Anakin skywalker yet...Then, between ANH and ESB, Lucas had this idea of Vader being Luke's dad...Personally, I believe that Ben was his real name and ObiWan was a name given to him by the Jedi. Of course, that idea has been destroyed by the prequels, but I don't watch the prequels so it's ok.
In fact, in the original draft of the script, they're two separate characters....
"Annikin Starkiller, a tall, heavy-set boy of eighteen..."
"Darth Vader, a tall, grim-looking general"