What causes my carpet in the traffic areas to look so dark

  The reason the carpet could look different in the traffic areas is because it just might be dirty and need to be cleaned or the carpet might be worn (carpet abrasion). The carpet fibers might have lost the twist and the fibers could be scratched and frayed. In this case the light will reflect differently and not look the same when you are looking at it. It is like, if you took sand paper to your car, the light would reflect in different directions and look darker and dull (not shiny). In the picture below, this is the same carpet about a foot apart from each other but one area is worn and the other area is not. The carpet fibers have loss their twist and they are frayed at the end and the carpet fibers are scratched making it look dull and not shiny. This happens more on Polyester carpet  and less on Nylon carpet. You will see this a lot in front of a sofa or on the bull nose or edge of the stairs where everyone slides their shoes. It is like an old pair of jeans, we can make it clean, but we can not take away the ware in the knees.

              It also could be Pile Reversal or (watermarking, pooling, shading) that might be happening to your carpet. This is when the carpet starts to go a different direction in the middle of the carpet or in the traffic areas where the foot traffic forces the pile to go in two different directions. This is really common on dense low cut pile carpet where the traffic is high.