I dont think its the hipsters who have kept WalMart out of urban areas as much as the difficulty in finding land parcels large enough to build a prototypical WalMart store.
http://www.columbia.edu/~pi17/walmartajs.pdf
Pretty much explains how WalMart got to be a rural business.
Property taxes likely have something to do with it, too. In more ways than one.
And shrinkage.