Milwaukee Journal Sentinel columnist Craig Gilbert speculates.
The theories are: 1. Trump lacks the "niceness" that's the style in Wisconsin. (But Trump does do well in other niceness-oriented midwestern states). 2. Scott Walker, despite his withdrawal, is still affecting Wisconsinites. 3. Local talk radio is attacking Trump. 4. Wisconsinites are more politically aware and engaged and thus less affected by the name recognition factor.