I definitely do NOT believe in ONLY buying "locally in-season". Nice fantasy, if you never want to eat lettuce, tomato, etc, in the winter above the frost line. But when it is good, and it is near-by, why can't it be in the local stores?
Multiple possible reasons. One is that a grocer needs a regular and reliable supply at a wholesale price. Local or afar doesn't change that. So, if they are to keep, for instance, local tomatoes in season as their primary tomato line, they need a supplies that can provide enough quantity, as often as needed. That means a competent grower, not an amateur who may or may not have a reliable crop. And it means that the local grower has to have a large enough and efficient enough operation to sell at grocer wholesale, not farmers market retail.
Another is that it takes more effort to buy locally. Standard produce is had from large wholesalers who supply most everything from everywhere with one point of contact. Local buying requires more calls and contacts to accomplish and more chances that something will happen, like the local guy gets sick, so no cucumbers that week. Grocers can't work like that. And the grower may well have to be able to present an invoice and wait for corporate to pay, just like any business grower.
Be clear. When we say "local produce," we don't mean the products of a bunch of backyard farmers or part-time country gardeners. We mean serious, professional local growers of substance and expertise. It's not just lack of stores selling local - it's lack of local farmers. It may be in season, but is there a large local grower of that vegetable so it can be had in commercial quantities in season?
My local chain is at least trying. They buy as locally as possible. I may be 200 miles away, but it's still in-state, and there's nothing closer. They also, to their everlasting credit, have small displays of things like heirloom tomatoes in season, grown locally, like within 20 miles of the store. I suspect they lose money on them from spoilage, but they're trying. So desire on the part of the sore has to be there, too. Since they're usually large corporate chains, it has to start in the corporate office with at least giving the produce managers authority to buy local goods when they can.
And stores are sensitive to customers. Ask the produce manager where things come from. Ask if they can't be had closer to home. If not, why not? Who at corporate decides? How many other customers can you get to ask the same questions? It's not produce, but a while back, my chain changes fresh tuna suppliers, and the result was a serious drop in quality. Enough people demanded answers that we're not back on the old supplier at the same good quality. It was strictly negative customer comment going back up the line through the seafood manager.