Why is early-season nitrogen fertilization not a common practice on soybeans?

The concern is that nitrogen applied during vegetative stages in soybeans may prevent symbiotic nitrogen fixation from fully developing. Pre-plant and late-season ammonium sulfate fertilization, on the other hand, may be beneficial by promoting and supplementing, respectively,

My soybeans are six to eight inches tall. Would it be okay to topdress them with 100 pounds per acre of ammonium sulfate?

While ammonium sulfate fertilization is often recommended pre-plant on low organic matter soils and, more recently, also late-season (R3) on high-yield potential soybeans, early-season nitrogen fertilization is normally only recommended in cases of poor nodulation.