Wal-Mart has buying power like no other... and they don't carry the same brands that most Quilt Shops do.
Also... when considering the price difference between Wal-Mart and your LQS, you won't get as much advice and help from most Wal-Mart fabric departments as you will from your local store. That "extra staffing" does add into the cost of the fabric.
Plus... remember when Wal-Mart started closing their fabric departments a couple of years back? You know if the fabric department was highly profitable at the prices Wal-Mart was charging, they would never have done away with fabric. That should tell you something, too.