Here's the pure, straight truth. The real reason Obi Wan didn't change Luke's last name is "Ben Kenobi." Obi Wan couldn't successfully come up with a new identity for himself, let alone Luke LOL. Like the name Kenobi isn't on the Empire's most wanted list. Giving Luke the last name of Lars would make the most sense, but the name Luke Lars is bad enough to keep the Tuskins away.