Because Milwuakee is not a very large metro area to begin with. Its on par with Providence RI, or OKC in terms of size. There is a misconception that its as big as Detroit, the Ohio C's, Indianapolis, or Pittsbugh/Philadelphia.
Milwaukee has a very tight metro definition. The Milwaukee CSA seems like a better representation of its size. Not including Walworth, Dodge, Racine, or Jefferson Counties takes the MSA definition down a lot. If Norman is part of the OKC metro, it feels like Racine should be of the Milwaukee Metro.
