The biggest reason was relative density of forces. After the initial war of maneuver on both fronts, a German division in the west would typically hold 2.5-3km of front, in the east 12-14km. And relatively similarly for their respective opponents, but just to illustrate that it wasn’t just a function of the relatively poorer scale of equipment of the Russians and Austrians compared to the French and British, it also applied to the basically same German forces on either front. This didn’t prevent significant offensive success in the west at times, in 1918 the Germans again pushed the Entente forces back significantly in France and then lost those gains and more before the Armistice, or static warfare in the east at times, but was probably the biggest factor in the greater tendency toward stalemate in the west.
Again though, such variations weren’t strictly limited to comparing the WW’s or within them. The latter phases of the American Civil War in the eastern theater showed a distinct tendency toward stalemates in entrenchments, and as mentioned before, the French made better progress in miles per day in Russia in 1812 than the Germans did in 1941, although the relatively few German armored and motorized divisions were disproportionately important in inflicting the huge losses suffered by the Soviets in the initial part of that campaign.