Lakers primarily use their length to defend inside the paint with Odom, Gasol and Bynum. The Mavs are primarily a jumpshooting team though that pass the ball around to open teammates. They usually have 3 or 4 good spot-up shooters in the game at a time to allow for better spacing. The Lakers just don’t match up well against the Mavs.
Same thing happened with the Spurs and Grizzlies. The Spurs were too old and not athletic enough to hang with the Grizzlies with their rugged inside game and tough perimeter defense. They are also giving the Thunder a run for their money with their 4-5 combo.