Space and number mostly. While the Germans had substantial forces committed to the Eastern front for the early portions of the war the balance of their forces were focused in Northern France from the English Chanel to just outside Paris (really almost a hinge point) down to the Swiss border. While this seems like a terrific distance, it is just a fraction of the size of the lines on the Eastern Front.

Once the Russians began to head toward Prussia, Germany needed to pull some of it's Army of the Western Front to counter. The German High Command knew in advance that such a thing might happen and had carefully planned tables and logistics to pull off the move in the shortest time possible. They also realized that they would need to "dress" their lines to prevent forces from being cut off and so behind many of these pockets they began digging the first "permanent" trench systems of the war. They then pulled back/adjusted the lines. Once it proved that these fortifications were going to be difficult to take, the allies started digging in on their side. The calcified lines didn't move much for the duration of the war from that point.

On the Eastern Front, the sheer manpower to length of lines really prevented large densities of people/trenches from forming. There were some terrains that made trenches impossible, and there were some localized pockets of heavy defensive fortifications, but they did not extend the literal length of the front like the West.