Does somebody know WHY exactly Peyton and Lucas left end of season 6?

To start a brand new life in a new place...
I think there wasn't any really logical explenation in the plot, it just because Chad (the acator who plays Lucas) didn't renewed his contract for the next season, so the screenwriters wanted to get this couple a closure with happy ending. That's why they made Lucas & Peyton leave Tree Hill (and the show).