I think there are many reasons and the way Jenson has brought the team together around him is going to have made a huge impact. He's quick, experienced and can give good feedback, very marketable (seen these days as the nice guy, never really says anything controversial and doesn't seem to get 'moody' when things aren't going perfectly)..all things that the team are going to want. Over the course of this season, Jenson has seemed much, much closer to team personnel and his race engineers especially, whereas Lewis has seemed more distant.
There's also one other thing that I think helps Jenson and hinders Lewis. Since winning the title in 2009, Jenson has looked far more laid-back and relaxed, and even stated it himself. The pressure is off him now, he's got the title and his new goal (the reason he said he went to McLaren) was to pit himself against the top-tier drivers and prove he could compete. In 2010, he was
very close to Lewis (and I feel he should have been closer in the final standings, but that's another discussion entirely) in what was Lewis' team, a car that was designed for a style far more aggressive than his, proving wrong those in the media who said that he would be blown away. On the other hand, Lewis puts himself under huge pressure. He wants more titles, everyone expects more titles from him and he wants to win at all costs. He sees Seb dominating in the Red Bull, a guy who he beat back in their junior years, and is frustrated because he feels he should have a car like that, able to fight for the win and the title at every event. He's not getting that at McLaren, even in 2008 it was a struggle and one wonders what might have happened if Kimi hadn't had the issues he did. So I feel that, along with the way Button is making the team his (I think if next year was to go the same as this year, the team would start looking to him as the natural leader (note that I don't mean #1 by this), is a major contributing factor to Lewis' unhappiness.
Edit: Uh. I just read that back to myself and it makes my brain hurt...but I can't be bothered fixing it.
