Very complex issue, but two of the major contributory factors were:
1) Numbers involved. Previously, most wars had tended to involve smaller, professional armies, usually with only two countries involved. The First World War was the first to involve coumtries from all corners of the globe and to involve huge conscripted armies.The sheer numbers involved made it very difficult for either side to achieve total victory.
2) Technology. The arms industry had developed significantly in the previous fifty years, particularly in respect of machine guns and artillery, so each side was very efficient at killing anyone who popped up in their sights. Unfortunately, the technology for moving people and equipment around at speed had not developed at the same pace. The development of the tank and aircraft in the 20s and 30s would make it easier for armies to turn superior resources into victories, but between 1914-18 the lack of mobility allied to huge superiority enjoyed by defensive armaments meant that the only alternative to suicidal frontal attacks was to dig in and try to wear out your opponent. Lack of imagination on the part of the commanding officers no doubt played a part.
A bit simplistic, but hope it gives some useful pointers.