Why did the western front barely move throughout world war 1?

In the western front, they used Trench Warfare Method 

In Trench Warfare the opposing teams would build some sort of trench surrounding their defended area to prevent the the opposing team in invading their protected Area. It's best used as a Defensive method which require the armies to stay hold and defended their position. That's why they're not moving so much 

Master planned communities outside of chicago. note: population density means people living in an area per square mile. chicago, an urban and highly populated area has a much more higher density than a desert, amish community, settlement, or nomadic region.