SpenceEdit said:
Plus if he had told Luke this in the first place, would Luke have gone with him? Would he have tried to reason with Vader and be killed, thus ending their last hope at restoring balance? If I were Obi-Wan, I wouldn't have told him till he was ready, and unfortunately he found out from Vader himself long before that time.
It's also worth noting how much Luke didn't know. Dude didn't even know what the Force was, probably didn't know what a Jedi was and didn't know his dad was one. Add on that there's a Dark Side that made his dad evil and that kind old Ben tossed him in a volcano and he's now the evilest man alive... .... talk about a mind blowing experience. If I was Luke I'd go to the cantina and drink myself to oblivion.