I'd put it down to the amount of stupid goals we used to concede. He's still better than anyone we have, and a better player than Fellaini, we need someone else to take the creative load off of Santi, he was poor in big games last season because teams knew that if they concentrated on neutralising him there weren't all that many others who could play a killer ball.
Mostly though it's down to the fact that we can buy Cesc for less than anyone else can, due to the 50% sell on clause in the deal Barcelona would make more money out of selling him to us for 25 million than they would from selling him to United for 49. It's all pie in the sky though, like I said I don't think he'll leave, not this summer anyway, if he ended up playing second fiddle to Thiago Alcantara in 2013/14 then maybe next year he might. I'd be delighted to sign Fellaini.
I hear you but not really an absolute to say that Cesc is "better" than Fellani, they're completely different players, Fellani is the type of player that Arsenal haven't had probably since Viera, Cesc is a luxury really whereas Fellani is a nesccesity. I mean on paper Cesc is a better player, better range of passing, great feet good pace, but Fellani is stronger, tougher in the tackle holds midfield together better, never goes missing, does the donkey work and covers the more ground than (I think) everyone except Michael Carrick. He also offers an entirely new aspect to Arsenal if they need it, he is the out ball, he is the "plan B" player that they haven't had for years. For that alone I'd say if you look at the options he brings you've got Arteta, Wilshire, Cazorla or Rosicky with him in behind for games against the top sides and if you have to go and batter a way through Stoke then in comes Ramsay into the holding role and play him in behind Theo Walcott. That alone is an enormous step forward for Arsenal. Cesc is basically more of the same only better, but it doesn't solve the big problems.
Next stop has to be securing a top drawer partner for Koselny. I'd go italian, any one of about 4 from Juve.