Another reason they were so successful farther West was simply the calibre of soldiery. Most Western armies were small professional cores surronded by a much larger ratio of levies - those levies were generally the ones used to weaken the enemy. You stick a large, heavy wagon in front of them and they immediately lose their capacity to aid the professional core.
During this same time in the East, most semi-nomadic peoples and horse-borne peoples are as much soldiers as they are herders, so they didn't run into this same issue quite so much. The more sedentary peoples, however, found that wagenbergs were useful against said peoples and didn't require as much training.