It wouldn't be very scary if the droid landed at the other side of the planet now would it?
When Luke says "What's wrong, you smell something?" you think the tauntaun smells the droid, which makes the appearance of the wampa much more of a surprise - at least I thought so the first time I saw ESB.
If you want an explanation for something that really doesn't need explaining I'll make up a story: the droid's sensors were all messed up due to the inpact - calibrating would take another 30 minutes. During calibration it just went a random direction - away from where Luke was.
How's that? :p