Why is a laser scanner not able to read a bar code on my iPhone?

So, I got this discount coupon via email. I printed it out as usual. But since I find it kind of inconvenient having to print coupons, I decided to try an experiment. When I went to pay for my purchase, I asked the clerk to try scanning the coupon on my iPhone. I did not work. But scanning the printed coupon worked just fine. Why is that?