I don't know whether the reason is that Ozil just doesn't suit the Premier League, eg a Veron, Shevchenko, Robinho type player, or he just doesn't fit in at Arsenal.
If Arsenal didn't have Ramsey and Wilshere then maybe he would get more scope to show what he could do but Wenger has congested Arsenal with a lot of attacking midfielders for a while.
Fergie pointed this out in his book, when he was talking about Arsenal's decline since 2004, when they needed to stiffen up the defensive areas of the team Wenger seemed curiously keen to splash a lot of Arsenals (limited) resources on collecting the same type of player: Rosicky, Ramsey, Nasri, Arshavin, Cazorla and now Ozil.
With these signings he was taking a gamble on whether they would fit in to the Premier League: Nasri and Ramsey yes, Rosicky, Cazorla OK, Arshavin, Ozil no.
The big irony was how Wenger responded when he had the chance to sign Fabregas for £27m. He couldn't have had any more certainty about how Fabregas would fit in to either the Premier League or Arsenal but that was the one he passed over.
Now all the speculation at Old Trafford is that Mata is going to be made available in the January transfer window. I wouldn't be entirely surprised to see him turn up at the Emirates.