He told om.net: "We had the chance of replying to the offers of other clubs, like Manchester (City), but he as well as me said 'no'.
"With Sunderland, it isn't an impulsive thing. But as he came to the end of this transfer window, he wanted to leave and showed a massive desire to play.
"It is that, more than his divorce from OM, that is in his mind at the moment."
The 27-year-old was not part of Les Bleus' squad for Euro 2008, and being regularly benched at OM was not helping his chances of returning to the France set-up.
"It is a chance for him to get his life back in order, as far as his career and as far as the France team is concerned," Diouf said.
"Everyone knows that his absence from the Euros mortified him. And he was fearing he would lose his place for good with Les Bleus if he stayed on the bench here.
"All these elements have played their part in this choice he has made, and to which we have responded favourably."