One issue is lag - the American general food supply diversifies more slowly than the population.
Another issue is corporate consolidation of the food supply chain. For example, New England used to have a lot of small confectionery manufacturers, from big factories to mom&pops, which over the last three decades have most all been crushed out by Nabisco. Which is much lower quality. So "why doesn't Boston have good confectionery like", err, Castile? Well, we used to be a lot closer, but Nabisco et al.