Print takes time to make and send out.
Online retailers can change their prices in seconds.
If Amazon gets ahold of Wal-Mart’s Black Friday sales flyer, they’ll adjust all their prices to be cheaper before Black Friday even comes around. A loss for Wal-Mart, and one they can’t pivot fast enough to avoid. It takes time to rearrange and reprint price tags. To update the computers. To update the flyer. Amazon can do all of that in a couple clicks.