I'm all for economic development and I'm certainly not against Walmart. But Walmart, as well as other big-box retailers, want it on the cheap. As an example, Walmart prefers to build on uninhabited land where there's no building to demolish, there's no trash or ruble to remove and there probably isn't any "environmental" remediation that needs to be done.
So, rather than building on property that already has exiting structure that is either out-dated or empty, Walmart prefers to build where it will be cheapest and fastest.
In Manassas, WalMart is building a store in the site of an old Sears on the corner of Manassas Mall. They had to tear down the old Sears building, re-locate a stream, do all new parking, and then connect their building into the existing mall structure.
Maybe there are other places Walmart prefers to build virgin, but they obviously are willing to recycle as well.