It's just a matter of different skill sets for different positions... Just like basketball, football, and any other sport...
The only reason A-Rod moved off of SS when he went to the Yankees was because Derek Jeter was the second coming of Jesus Christ in New York and Yankee fanswould have rioted had they asked Jeter to move... They'd have been better off with Alex at SS, but that's neither here nor there...
Dustin Ackley won't have to play the outfield in the bigs, but if they think he's mobile and athletic enough to play there, he's gotbetter value out there... First base is generally reserved for your guys that flat out cannot play anywhere else on the MLB level... Just less field to coverthere than anywhere else, and mobility/athleticism/arm strength aren't as much an issue as they would be anywhere else...
Up the middle (SS, 2B, CF) has always been more about speed and range than anything else... There's a ton of ground to cover at those three positions,because you've got a lot of field going both to the left and right of those spots... You generally put your best athletes in one of those three spotsdefensively until they prove they can't handle it... You put smaller, faster guys at those three positions, and your expectation for power production goesdown... It's just always been more about defense at those 3 spots than offensive value, up until recently, because those three positions can essentiallymake or break your team defensively...
Third base you want quick reactions, sure hands and a strong arm before anything else... Lots of hard hit balls down there, thus the 'hot corner', soguys need to be able to react to balls quick... Longest throws in the infield... Range isn't as much an issue, more a luxury there than anything else...
The corner outfield (LF and RF) are like first base in that they're essentially the last place you stick guys because they're relatively easy to play,compared to everywhere else... Only real thing here is you have to have guys with good arms, because they're making the longest throws on the field...