Simple reason : ANH was made back in 77 ... Lucas didn't have the WHOOOOOLLLLE story down to the exact details ..... he didn't know he'd be filming episodes 1 and 2 two whole decades later ... leave alone that ... he didn't even know if he would be filming episode 5 ... therefore at that point, I doubt he had thought of details like why they would name him "Skywalker" AND put him on Tattoine ... because that doesn't make logical sense ... if they were trying to hide him from Vader, they couldn't have done a worse job. Point being, when you're trying to thread together a storyline that's as long and involved as this one, with a gap of 20 years in between making movies, sometimes you'll encounter these kinda bloopers .... Luke Skywalker obviously sounded a lot better than Luke Lars in 77 ...