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.
From my own experience Racine sort of feels like it's own thing. Milwaukee is a bit weird because Density drops off very fast as soon as you leave Milwaukee County and you have a lot of dense exurbs - just how far you should let that boundary extend is a difficult question in this case.
