See these pictures of the Husky liners: Husky Liners
See how the mat covers up high on the left side of the driver's mat, near the hood release, where your left foot would rest? This is the "dead pedal" area. The Weathertech mats only cover about half of that area, so if you rest your foot there, half of your foot will be resting on carpet rather than the liner.
Compare those Husky pictures to this picture of the Weathertech, and hopefully you'll see what I mean: