CO, first of all they did not just suddenly come up with the idea of a bull/calf. It was the most prevelent symbol of religion in the areas in which they travelled. so when they did not see the power of this strange God who was leading them blindly through strange lands, they longed for the comfort of something tangible they could relate to at first glance.
No different today that people would not suddenly bow to a strange idol. Rather, in the case of the holdeman followers, they do not see signs of a God or Spirit which they are directed in writing and prose to worship. There are no miraculous happenings, healings, or many dramatic conversions from troubled lives. (well, there are stories from "somewhere else")
So the people long for a religious element they can recognize at first sight (think of their 'sisters' dress code for example) and this becomes a fixation, a familiar and comfortable sign of religion. The uniformity of "our schools", programs, and the leadership which comforts (by chastisement) like the proverbial shephards rod becomes a tangible "god" or religious icon they can be obedient to and follow.
Just as the Isrealites of old did not create a new icon, but built a familiar one that gave instant recognition, unity, and comfort; so the holdeman people have built an edifice of sorts to comfort them as they travel through "the world" like "pilgrims and strangers" (albeit in comfort not on camels and donkeys).