Mainly because of the speed with which you can make the transfer.  You can pull out paper bills and pay in single-digit seconds, just like cash.  When everybody has a smartphone and could pull it out and wave it and pay with under 10 seconds, my proposal will be meritless.  But not everybody has a smartphone, and those that do will spend 30-60 seconds opening the app, scanning a code, initiating a transaction, and waiting for the merchant POS to detect the transaction, and those seconds are very long when you're holding up the line at the checkout counter figuring out your Bitcoin payment and wishing you just swiped your debit card.