Because he is a stubborn old fool. The team should be a fluid 4-3-3, and, despite leaving behind Andy Johnson and Theo Walcott, he has the players at his disposal. Barry should be the deep lying midfielder, giving Lampard licence to bomb forward the way he does at Chelsea. In the current formation, he plays 90% of the match with his back to goal. Gerrard should be given freedom to roam, linking up with Rooney (who, by the way was a disgrace last night). It means the wide men (and I would pick Joe Cole and SWP) would need the discipline to help the defence as well as attack. That shouldn't be a problem because the two Cole's are used to that at Chelsea. For the Slovenia game (and I can't believe that we have to win to qualify from a group which many of us would have happily hand-picked), my line-up would be: James, Johnson, Dawson (oh dear), Terry, Cole, Barry, Lampard, Gerrard, SWP, Cole, Rooney.
In simple terms, play the formation the players are comfortable with.