This one is hard to reconcile when you think about it. I guess that stems from Yoda's feeling that Luke's 'family' is the best place for him (and not Obi-Wan). Of course, this is after he lets Bail Organa take Leia.

My feeling on 'why' is that Obi-Wan explained (offscreen)the situation to Owen, and the importance of keeping Luke on a short leash with instructions to give Luke very little information about his biological parents and the events leading him there, hoping to keep him low key. After Owen and Beru are briefed and agree to take Luke, we see the handoff at the end of the film.

I know, not great, but plausible.