As a non-American I am less than qualified to provide an answer, but one possible hypothesis is Race and ethnicity.
The midwest has according to Wikipedia with 79% the highest Non-Hispanic White ratio of any region.[1] As Blacks have the highest rate of poverty 27%, followed by Hispanics 25% compared to 10% for non-Hispanic Whites. [2]
Income equality might be explained by having less poor people.<p>A follow up hypothesis would be that racial homogeneity might lead to less ghettoisation resulting in less competition for the lowest paying jobs. One might feel less compelled to pay only the lowest possible wage if one can relate to the other person, by race, neighbourhood, etc.<p>Now, as to why Blacks and Hispanics have higher poverty rates; one explanation may be that the American social system relies very heavily on both parents present (and working) for a child to grow up in an advantageous situation. This is something I have observed to a much lesser extend in more socialist societies in Europa. And famously in all but 11 states, most black children do not live with both parents. In every state, 7 in 10 white children do. [3]<p>[1] <a href="https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Race_and_ethnicity_in_the_United_States" rel="nofollow">https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Race_and_ethnicity_in_the_Unit...</a><p>[2] <a href="http:&#x2F;&#x2F;www.csgmidwest.org&#x2F;policyresearch&#x2F;1012incometrends.aspx" rel="nofollow">http:&#x2F;&#x2F;www.csgmidwest.org&#x2F;policyresearch&#x2F;1012incometrends.as...</a><p>[3] <a href="http:&#x2F;&#x2F;www.washingtontimes.com&#x2F;news&#x2F;2012&#x2F;dec&#x2F;25&#x2F;fathers-disappear-from-households-across-america&#x2F;?page=all" rel="nofollow">http:&#x2F;&#x2F;www.washingtontimes.com&#x2F;news&#x2F;2012&#x2F;dec&#x2F;25&#x2F;fathers-disa...</a>