It's simple:
#1 who has the stats to confirm superiority?
#2 Who's the most fun to watch?
#3 Who is the most likely player that can turn a bad day into a winning day?
While is true that Iniesta has great skills and has made much merit not only with Barca, but with his national team, he's not the best player during 2012; not better than Messi.
Messi has the edge.
Numbers:
#1 Messi has scored 26 La Liga goals to Iniesta's 1.
#2 Messi has scored 5 CL goals to Iniesta's 1.
#3 Messi has just surpassed the all-time record of goals scored by a player in one year, at 91.
While it is true that Iniesta was named best player at the recent Euro, he didn't score any goals there, while Messi scored 5 goals with his national team. Not the same, but a good reference.
Perception:
* Messi attracts defenders galore, even without the ball and this helps his temmates by reducing the defenders around them
* Messi is fun to watch; you can expect from him someting phenomenal at any time.
* Messi carries Barca. He has won a significant number of games all by himself.
* The question nobody likes to ask is: How much of Barca's recent glory would have been possible without Messi?
The whole world knows of Barca's achievements and of Messi's incredible contribution to the results, which this year has made an even bigger impact than ever. How can Iniesta or Ronaldo come close to match it? Has Ronaldo proven an influence to Madrid that comes even close to Messi's to Barca?
Just because Iniesta and Ronaldo are great players does not mean that they should win the Balon D'or.
Being good, or great, is not enough to win the Balon D'or.
Being superior to the rest, where it counts, does. Neither Ronaldo, nor Iniesta have had a bigger influence than Messi.