I feel like a better question is why did the Democrats lose votes in BOTH Milwaukee among African-Americans AND more rural / small-town regions. Because Democrats need both those voters to win.
The Milwaukee County loss in turnout wasn't that substantial when you consider some of the suburbs swung to the left. The real killer that nobody saw coming (or to the extent, at least) was all the of the backlash in rural Wisconsin. No polls detected that, but really I think the last few weeks killed Clinton, because she didn't visit the Midwest nearly as much as she should have.
