Because Wenger didn't think Modric could cut it physically in the Premier League.
Watching him keep hold of the ball whilst bouncing off meaty challenges from all angles, (with Djorou resorting to scything him down unceremoniously in the second half), dusting himself down and getting on with it MUST have grated on Arsene. Given how he ended up with Rosicky and Arshavin - 2 similar players who are completely incapable of coping with the demands of English football, one a perma crock and the other as lazy as they come.
That is certainly one Wenger fucked up royally. And signing Diaby