Much like in the NFL you draft tackles because if they can't make it at tackle, they can move to guard and be decent players. See Gallery, Robert of the Raiders.
You don't draft 2nd basemen highly in the draft. Why? Because a SS who may not have the range to play SS in the majors can move to 2nd. But a 2nd who is a bust at 2nd, is basically screwed.
And please don't give me the BS about Pedrioa. I'll list at least five players drafted as 2nd, who were busts.