Two main factors contribute to this lag, said study author and market analyst Steve Caine, a partner with Bain & Company. First, shoppers have been reluctant to entrust their grocery selection to someone else. Second, they don't like the idea of paying extra for the service. 

"Amazon has trained us not to pay for services. Even though we may pay a Prime membership, it feels like everything just comes for free," said Caine. "So for someone to say it's going to be a $7 or $10 delivery charge, that's actually a pretty big deterrent."