The Germans were intent to stay so they reinforced their positions with concrete (also on the Western front). The Allied side was intent on moving them out as fast as possible, so positions were considered temporary and generally not reinforced. Of course it never turned out that way, but then neither side was guilty of learning much during those 4 years.

The effect can still be seen today in the western corner of Belgium where some German positions are intact while most Allied positions are little more than overgrown ditches.